[core/errors] Add custom exceptions

Add custom exceptions and add error handling to the engine's module
loading logic. I.e. when a non-existent module is loaded, an exception
is thrown now.

see #23

bumblebee-status

@@ -16,6 +16,10 @@ def main():
     engine.run()
 
 if __name__ == "__main__":
-    main()
+    try:
+        main()
+    except bumblebee.error.BaseError as error:
+        sys.stderr.write("fatal: {}\n".format(error))
+        sys.exit(1)
 
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