Catch KeyboardInterrupt so we don't barf a trace

Concerns issue #1531.

Future work can actually try to save state in this case, but especially
with multiprocessing, that might become a bit hairy.

overviewer.py

@@ -659,3 +659,6 @@ See http://docs.overviewer.org/en/latest/index.html#help
 
 This is the error that occurred:""")
         util.nice_exit(1)
+    except KeyboardInterrupt:
+        logging.info("Interrupted by user. Aborting.")
+        util.nice_exit(2)

overviewer_core/dispatcher.py

@@ -261,6 +261,8 @@ class MultiprocessingDispatcherProcess(multiprocessing.Process):
                 self.result_queue.put(result, False)
             except queue.Empty:
                 pass
+            except KeyboardInterrupt:
+                return
 
 class MultiprocessingDispatcher(Dispatcher):
     """A subclass of Dispatcher that spawns worker processes and