Display exception string when loading a plugin failed

This helps with seeing what the issue is (e.g. with django-stubs [1]).

It might be even better to have a traceback there already, no?

1: typeddjango/django-stubs#160

Author: blueyed

mypy/build.py

@@ -381,8 +381,8 @@ def plugin_error(message: str) -> None:
 
         try:
             module = importlib.import_module(module_name)
-        except Exception:
-            plugin_error("Error importing plugin '{}'".format(plugin_path))
+        except Exception as exc:
+            plugin_error("Error importing plugin '{}': {}".format(plugin_path, exc))
         finally:
             if plugin_dir is not None:
                 assert sys.path[0] == plugin_dir

test-data/unit/check-custom-plugin.test

@@ -70,7 +70,7 @@ tmp/mypy.ini:2: error: Can't find plugin 'tmp/missing.py'
 [[mypy]
 plugins=missing
 [out]
-tmp/mypy.ini:2: error: Error importing plugin 'missing'
+tmp/mypy.ini:2: error: Error importing plugin 'missing': No module named 'missing'
 
 [case testMultipleSectionsDefinePlugin]
 # flags: --config-file tmp/mypy.ini