Handle cases when inspect.stack() fails

Ricardo @Panaggio reported an error: creating BaseSpider class fails if the first import is inside jinja2 template. Stacktrace:

```
Traceback:
File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py" in get_response
  111.                         response = callback(request, *callback_args, **callback_kwargs)
File "/home/ricardo/demos/tmp/shub/web/utils/apikeys.py" in wrapped
  38.         return view_func(request, *args, **kwargs)
File "/home/ricardo/demos/tmp/shub/web/utils/httpauth.py" in wrapped
  22.         return view_func(request, *a, **kw)
File "/usr/lib/python2.7/dist-packages/django/contrib/auth/decorators.py" in _wrapped_view
  20.                 return view_func(request, *args, **kwargs)
File "/home/ricardo/demos/tmp/shub/web/utils/hs.py" in wrapper
  26.         return func(request, *args, **kwargs)
File "/home/ricardo/demos/tmp/shub/web/utils/projects.py" in wrapper
  62.         return view(request, *args, **kwargs)
File "/home/ricardo/demos/tmp/shub/web/utils/decorators.py" in wrapper
  156.                                       mimetype=mimetype)
File "/usr/lib/python2.7/dist-packages/django/shortcuts/__init__.py" in render_to_response
  20.     return HttpResponse(loader.render_to_string(*args, **kwargs), **httpresponse_kwargs)
File "/usr/lib/python2.7/dist-packages/django/template/loader.py" in render_to_string
  176.         return t.render(context_instance)
File "/usr/lib/python2.7/dist-packages/django_jinja/base.py" in render
  132.         return super(Template, self).render(new_context)
File "/usr/lib/python2.7/dist-packages/jinja2/environment.py" in render
  894.         return self.environment.handle_exception(exc_info, True)
File "/home/ricardo/demos/tmp/shub/web/templates/projects/project_spiders.jinja" in top-level template code
  3. {% import "macros/csrf.jinja" as csrf with context %}
File "/home/ricardo/demos/tmp/shub/web/templates/base_projects.jinja" in top-level template code
  1. {% extends "base.jinja" %}
File "/home/ricardo/demos/tmp/shub/web/templates/base.jinja" in top-level template code
  2. {% import "macros/js.jinja" as js with context %}
File "/home/ricardo/demos/tmp/shub/web/templates/base_site.jinja" in top-level template code
  27.             {% block body %}{% endblock %}
File "/home/ricardo/demos/tmp/shub/web/templates/base.jinja" in block "body"
  118.             {% block content %}
File "/home/ricardo/demos/tmp/shub/web/templates/base.jinja" in block "content"
  122.                 <div class="tabs-content">{% block tabs_content %}{% endblock %}</div>
File "/home/ricardo/demos/tmp/shub/web/templates/projects/project_spiders.jinja" in block "tabs_content"
  61.                                 <a href="{{ job_url(stat['key']) }}#items">
File "/home/ricardo/demos/tmp/shub/web/utils/jinja2compat/functions.py" in job_url
  15.     return reverse('job:details', args=(projectid, spider, jobid))
File "/usr/lib/python2.7/dist-packages/django/core/urlresolvers.py" in reverse
  476.     return iri_to_uri(resolver._reverse_with_prefix(view, prefix, *args, **kwargs))
File "/usr/lib/python2.7/dist-packages/django/core/urlresolvers.py" in _reverse_with_prefix
  363.         possibilities = self.reverse_dict.getlist(lookup_view)
File "/usr/lib/python2.7/dist-packages/django/core/urlresolvers.py" in reverse_dict
  276.             self._populate()
File "/usr/lib/python2.7/dist-packages/django/core/urlresolvers.py" in _populate
  253.                     for name in pattern.reverse_dict:
File "/usr/lib/python2.7/dist-packages/django/core/urlresolvers.py" in reverse_dict
  276.             self._populate()
File "/usr/lib/python2.7/dist-packages/django/core/urlresolvers.py" in _populate
  265.                 lookups.appendlist(pattern.callback, (bits, p_pattern, pattern.default_args))
File "/usr/lib/python2.7/dist-packages/django/core/urlresolvers.py" in callback
  216.         self._callback = get_callable(self._callback_str)
File "/usr/lib/python2.7/dist-packages/django/utils/functional.py" in wrapper
  27.         result = func(*args)
File "/usr/lib/python2.7/dist-packages/django/core/urlresolvers.py" in get_callable
  92.                 lookup_view = getattr(import_module(mod_name), func_name)
File "/usr/lib/python2.7/dist-packages/django/utils/importlib.py" in import_module
  35.     __import__(name)
File "/home/ricardo/demos/tmp/shub/web/apps/job/views.py" in <module>
  15. from shub.web.utils.items import maybe_extract_items, get_schemas, \
File "/home/ricardo/demos/tmp/shub/web/utils/items.py" in <module>
  2. from slybot.spider import IblSpider
File "/usr/lib/pymodules/python2.7/slybot/spider.py" in <module>
  9.     from scrapy.spider import Spider
File "/usr/lib/pymodules/python2.7/scrapy/spider.py" in <module>
  68. BaseSpider = create_deprecated_class('BaseSpider', Spider)
File "/usr/lib/pymodules/python2.7/scrapy/utils/deprecate.py" in create_deprecated_class
  98.     frm = inspect.stack()[1]
File "/usr/lib/python2.7/inspect.py" in stack
  1054.     return getouterframes(sys._getframe(1), context)
File "/usr/lib/python2.7/inspect.py" in getouterframes
  1032.         framelist.append((frame,) + getframeinfo(frame, context))
File "/usr/lib/python2.7/inspect.py" in getframeinfo
  1007.             lines, lnum = findsource(frame)
File "/usr/lib/python2.7/inspect.py" in findsource
  580.             if pat.match(lines[lnum]): break

Exception Type: IndexError at /p/1844/spiders/
Exception Value: list index out of range
```

`lines` list contain lines of jinja2 template, not the python source code in this case, and it looks like inspect.stack() gets confused. In this PR such errors are catched and converted to warnings.