Code review comment for lp:~newell-jensen/maas/fix-1553841

Code looks fine to me. Just a couple things I'm wondering:

 * Can you update the commit message so that it is more accurate? (it makes it sound like error propagation in general, but the branch seems to just fix up the AMT exceptions so that they are thrown correctly.)

 * Are we confident the AMT utilities will always print these particular strings? (I assume we're using a consistent locale, so that the strings don't show up translated into another language and surprise us. And I was wondering if using just the HTTP status code for matching might be more future-proof... assuming the output isn't too large, and we could match the number in multiple places in the string...?)