Merge 461960-update-error-wrapper [r=free.ekanayaka] [f=461960]
This branch refactors the error handling for txAWS, not only addressing the
original scope of the ticket (handling ConnectionRefusedError) but also
adding support for handling Amazon-originated 500-errors, non-AWS 400 and 500
errors, and handing improperly configured endpoints that are returning HTML.