Handle missing default formatter

In the event that we cannot load our plugins, we shouldn't raise a
cryptic KeyError from our formatter.

Closes #320

src/flake8/exceptions.py

@@ -13,6 +13,10 @@ class EarlyQuit(Flake8Exception):
     pass
 
 
+class ExecutionError(Flake8Exception):
+    """Exception raised during execution of Flake8."""
+
+
 class FailedToLoadPlugin(Flake8Exception):
     """Exception raised when a plugin fails to load."""
 

src/flake8/main/application.py

@@ -195,9 +195,19 @@ class Application(object):
             elif 2 <= self.options.quiet:
                 format_plugin = 'quiet-nothing'
 
+            try:
+                default_formatter = self.formatting_plugins['default']
+            except KeyError:
+                raise exceptions.ExecutionError(
+                    "The 'default' Flake8 formatting plugin is unavailable. "
+                    "This usually indicates that your setuptools is too old. "
+                    "Please upgrade setuptools. If that does not fix the issue"
+                    " please file an issue."
+                )
+
             if formatter_class is None:
                 formatter_class = self.formatting_plugins.get(
-                    format_plugin, self.formatting_plugins['default']
+                    format_plugin, default_formatter
                 ).execute
 
             self.formatter = formatter_class(self.options)
@@ -332,6 +342,11 @@ class Application(object):
             LOG.exception(exc)
             self.file_checker_manager._force_cleanup()
             self.catastrophic_failure = True
+        except exceptions.ExecutionError as exc:
+            print('There was a critical error during execution of Flake8:')
+            print(exc.message)
+            LOG.exception(exc)
+            self.catastrophic_failure = True
         except exceptions.EarlyQuit:
             self.catastrophic_failure = True
             print('... stopped while processing files')