userscripts: Remove on_proc_error.

When there was an error, the finished signal will be emitted too anyways, so if
we call cleanup here, we'll call it twice which means we'll get an exception.

Supersedes #1175.

CHANGELOG.asciidoc

@@ -121,6 +121,7 @@ Fixed
   (U+2028/U+2029/BOM).
 - Movements in caret mode now should work correctly on OS X and Windows.
 - Fixed upgrade from earlier config versions.
+- Fixed crash when killing a running userscript.
 
 v0.4.1
 ------

qutebrowser/commands/userscripts.py

@@ -113,7 +113,6 @@ class _BaseUserscriptRunner(QObject):
         self._proc = guiprocess.GUIProcess(self._win_id, 'userscript',
                                            additional_env=self._env,
                                            verbose=verbose, parent=self)
-        self._proc.error.connect(self.on_proc_error)
         self._proc.finished.connect(self.on_proc_finished)
         self._proc.start(cmd, args)
 
@@ -158,10 +157,6 @@ class _BaseUserscriptRunner(QObject):
         """
         raise NotImplementedError
 
-    def on_proc_error(self, error):
-        """Called when the process encountered an error."""
-        raise NotImplementedError
-
 
 class _POSIXUserscriptRunner(_BaseUserscriptRunner):
 
@@ -202,10 +197,6 @@ class _POSIXUserscriptRunner(_BaseUserscriptRunner):
         """Interrupt the reader when the process finished."""
         self.finish()
 
-    def on_proc_error(self, error):
-        """Interrupt the reader when the process had an error."""
-        self.finish()
-
     def finish(self):
         """Quit the thread and clean up when the reader finished."""
         log.procs.debug("Cleaning up")
@@ -256,11 +247,6 @@ class _WindowsUserscriptRunner(_BaseUserscriptRunner):
         self._cleanup()
         self.finished.emit()
 
-    def on_proc_error(self, error):
-        """Clean up when the process had an error."""
-        self._cleanup()
-        self.finished.emit()
-
     def run(self, cmd, *args, env=None, verbose=False):
         try:
             self._oshandle, self._filepath = tempfile.mkstemp(text=True)