Added logfile logging for BAD REQUEST HTTP errors.

To better understand what went wrong with the request. (Redmine issue: #3443)

lib/hawat/app.py

@@ -190,6 +190,14 @@ def _setup_app_core(app):
     @app.errorhandler(400)
     def eh_badrequest(err):  # pylint: disable=locally-disabled,unused-variable
         """Flask error handler to be called to service HTTP 400 error."""
+        flask.current_app.logger.critical(
+            "Bad Request Error:\n%s",
+            ''.join(
+                traceback.TracebackException(
+                    *sys.exc_info()
+                ).format()
+            )
+        )
         return hawat.errors.error_handler_switch(400, err)
 
     @app.errorhandler(403)