Implement better UX around broken plugins

This handles most any exception arising from a plugin and provides the
user with a better experience and view of the problem. It also provides
a way to retrieve the information about what exactly failed for
providing bug reports to plugin authors.

src/flake8/checker.py

@@ -432,7 +432,15 @@ class FileChecker(object):
                 plugin=plugin,
                 exception=ae,
             )
-        return plugin['plugin'](**arguments)
+        try:
+            return plugin['plugin'](**arguments)
+        except Exception as all_exc:
+            LOG.critical('Plugin %s raised an unexpected exception',
+                         plugin['name'])
+            raise exceptions.PluginExecutionFailed(
+                plugin=plugin,
+                excetion=all_exc,
+            )
 
     @staticmethod
     def _extract_syntax_information(exception):

src/flake8/exceptions.py

@@ -76,6 +76,25 @@ class PluginRequestedUnknownParameters(Flake8Exception):
                               'exc': self.original_exception}
 
 
+class PluginExecutionFailed(Flake8Exception):
+    """The plugin failed during execution."""
+
+    FORMAT = '"%(name)s" failed during execution due to "%(exc)s"'
+
+    def __init__(self, *args, **kwargs):
+        """Utilize keyword arguments for message generation."""
+        self.original_exception = kwargs.pop('exception')
+        self.plugin = kwargs.pop('plugin')
+        super(PluginExecutionFailed, self).__init__(
+            str(self), *args, **kwargs
+        )
+
+    def __str__(self):
+        """Format our exception message."""
+        return self.FORMAT % {'name': self.plugin['plugin_name'],
+                              'exc': self.original_exception}
+
+
 class HookInstallationError(Flake8Exception):
     """Parent exception for all hooks errors."""
 

src/flake8/main/application.py

@@ -309,7 +309,12 @@ class Application(object):
         if self.running_against_diff:
             files = sorted(self.parsed_diff)
         self.file_checker_manager.start(files)
-        self.file_checker_manager.run()
+        try:
+            self.file_checker_manager.run()
+        except exceptions.PluginExecutionFailed as plugin_failed:
+            print(str(plugin_failed))
+            print('Run flake8 with greater verbosity to see more details')
+            self.catastrophic_failure = True
         LOG.info('Finished running')
         self.file_checker_manager.stop()
         self.end_time = time.time()