fixup! Display exception string when loading a plugin failed

Author: blueyed

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