Fixed #20167 -- Preserve the traceback of `ImportError`s in `import_by_path`.

Thanks @carljm for the review.

django/utils/module_loading.py

@@ -3,6 +3,7 @@ import os
 import sys
 
 from django.core.exceptions import ImproperlyConfigured
+from django.utils import six
 from django.utils.importlib import import_module
 
 
@@ -19,8 +20,10 @@ def import_by_path(dotted_path, error_prefix=''):
     try:
         module = import_module(module_path)
     except ImportError as e:
-        raise ImproperlyConfigured('%sError importing module %s: "%s"' % (
-            error_prefix, module_path, e))
+        msg = '%sError importing module %s: "%s"' % (
+            error_prefix, module_path, e)
+        six.reraise(ImproperlyConfigured, ImproperlyConfigured(msg),
+                    sys.exc_info()[2])
     try:
         attr = getattr(module, class_name)
     except AttributeError:

tests/utils_tests/module_loading.py

@@ -120,6 +120,16 @@ class ModuleImportTestCase(unittest.TestCase):
             import_by_path('unexistent.module.path', error_prefix="Foo")
         self.assertTrue(str(cm.exception).startswith('Foo'))
 
+    def test_import_error_traceback(self):
+        """Test preserving the original traceback on an ImportError."""
+        try:
+            import_by_path('test_module.bad_module.content')
+        except ImproperlyConfigured:
+            traceback = sys.exc_info()[2]
+
+        self.assertIsNotNone(traceback.tb_next.tb_next,
+            'Should have more than the calling frame in the traceback.')
+
 
 class ProxyFinder(object):
     def __init__(self):