Output stash errors as a WARNING log ouput instead of DEBUG

[Karl O]

If you're running the bot with log.INFO logging level you're probably still interested if Stash is down or not authenticating you.

Example:
bot_1 | 2017-02-14 08:02:04,450 root DEBUG: Parsing stash response: {'errors': [{'context': None, 'message': 'You are not permitted to access this resource', 'exceptionName': 'com.atlassian.bitbucket.AuthorisationException'}]}