Change invalid date error code to 400

The create lease API and the update lease API returned a 500 response
status code when the date format is invalid. But this should be
400 BadRequest as well as wrong time order (InvalidInput) exception.

This patch changes it to return a 400 response status code.

Change-Id: I7f259d06d8b5e0b901126064a21d313497721e1b

blazar/manager/exceptions.py

@@ -82,6 +82,7 @@ class EventError(exceptions.BlazarException):
 
 
 class InvalidDate(exceptions.BlazarException):
+    code = 400
     msg_fmt = _(
         '%(date)s is an invalid date. Required format: %(date_format)s')