New success/error handling for callbacks

The new `python-django-qmgmt-process-status-ok' buffer local variable is
set for process buffers and it's non-nil on success and nil on errors.

This allows for more adequate error handling for failed commands.
Better error reporting.

* python-django.el:
(python-django-util-shell-command-to-string): New function.
(python-django-util-shell-command-or-error): New function.
(python-django--help-get):
(python-django-info-get-version):
(python-django-info-get-settings):
(python-django-info-get-setting):
(python-django-info-get-app-paths):
(python-django-info-module-path): Use it.
(python-django-open-project): Print user errors to project buffer.
Generate readable code for python interactions
Merge branch 'better-error-handling'
Fix allowed file extension checks (Bug#12)

* python-django.el:
(python-django-util-file-name-extension): New function.
(python-django-ui-tree-dir-widget-expand): Use it.
Add os to `python-django-info-imports-code`
Import JSON properly for newer Django versions
Merge pull request #1 from fgallina/master

update