Show a nicer message when duplicate entry points are found

This removes the stack trace from the message printed, which is
somewhat more friendly, though still treats this misconfiguration
as a catastrophic error.

src/flake8/exceptions.py

@@ -18,7 +18,7 @@ class ExecutionError(Flake8Exception):
     """Exception raised during execution of Flake8."""
 
 
-class DuplicatePluginEntryPoint(Flake8Exception):
+class DuplicatePluginEntryPoint(ExecutionError):
     """Exception raised when a plugin entry point is already taken."""
 
     FORMAT = (

src/flake8/main/application.py

@@ -121,15 +121,18 @@ class Application(object):
         This should be the last thing called on the application instance. It
         will check certain options and exit appropriately.
         """
+        if self.catastrophic_failure:
+            # Don't rely on any attributes being set if things failued
+            # catastrophically
+            raise SystemExit(True)
+
         if self.options.count:
             print(self.result_count)
 
         if self.options.exit_zero:
-            raise SystemExit(self.catastrophic_failure)
+            raise SystemExit(False)
         else:
-            raise SystemExit(
+            raise SystemExit(self.result_count > 0)
-                (self.result_count > 0) or self.catastrophic_failure
-            )
 
     def find_plugins(self, config_finder):
         # type: (config.ConfigFileFinder) -> None

tests/unit/test_application.py

@@ -48,6 +48,18 @@ def test_exit_does_raise(result_count, catastrophic, exit_zero, value,
     assert excinfo.value.args[0] is value
 
 
+def test_exit_raises(application):
+    """Verify Application.exit raises SystemExit under configuration failure."""
+    application.catastrophic_failure = True
+    # Note: no application.options set -- configuration issues can lead to
+    # errors before it's assigned.
+
+    with pytest.raises(SystemExit) as excinfo:
+        application.exit()
+
+    assert excinfo.value.args[0] is True
+
+
 def test_warns_on_unknown_formatter_plugin_name(application):
     """Verify we log a warning with an unfound plugin."""
     default = mock.Mock()