import_by_path should show the original traceback on an ImportError

[grampajoe]

When django.utils.module_loading.import_by_path encounters an ImportError, it hides the original traceback:

>>> import_by_path('utils_tests.test_module.bad_module.content')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/grampajoe/Projects/django/django/utils/module_loading.py", line 28, in import_by_path
    error_prefix, module_path, e))
django.core.exceptions.ImproperlyConfigured: Error importing module utils_tests.test_module.bad_module: "No module named a_package_name_that_does_not_exist"

That gives us the original exception message, but the traceback isn't relevant or useful.

I've written a fix using six.reraise at ​https://github.com/grampajoe/django/tree/import-error-reraise.

[grampajoe]

Added a pull request with the fix: ​https://github.com/django/django/pull/979
Tests passing with SQLite.

[Simon Charette <charette.s@…>]

In 2d0db67813d457dc890a2287d9e98593565d650f:

Fixed #20167 -- Preserve the traceback of ImportErrors in import_by_path.

Thanks @carljm for the review.