catch exception for NoSectionError

Users don't like seeing tracebacks.

Before:
```
(env)$ supernova foo list
Traceback (most recent call last):
  File "/home/carl/Projects/github.com/supernova/env/bin/supernova", line 9, in <module>
    load_entry_point('supernova==1.0.1', 'console_scripts', 'supernova')()
  File "/home/carl/Projects/github.com/supernova/supernova/executable.py", line 84, in run_supernova
    snobj.run_novaclient(nova_args, supernova_args)
  File "/home/carl/Projects/github.com/supernova/supernova/supernova.py", line 101, in run_novaclient
    self.prep_shell_environment()
  File "/home/carl/Projects/github.com/supernova/supernova/supernova.py", line 92, in prep_shell_environment
    for key, value in self.prep_nova_creds():
  File "/home/carl/Projects/github.com/supernova/supernova/supernova.py", line 52, in prep_nova_creds
    raw_creds = config.nova_creds.items(self.nova_env)
  File "/usr/lib64/python2.7/ConfigParser.py", line 347, in items
    raise NoSectionError(section)
ConfigParser.NoSectionError: No section: 'foo'
```

After:
```
(env)$ supernova foo list
[Failed] Unable to locate section 'foo' in your configuration.
```