lp-propose-merge has poor error reporting

[Tristan Seligmann]

Running lp-propose-merge causes some other bzr commands/operations to be run "under the hood" in order to push the branch to Launchpad and such. If one of these operations fails, there is no way to tell what the failing operation was, and the error messages don't provide any assistance with this either. For example:

mithrandi@verdant:~/code/CENSORED> bzr lp-submit -m 'CENSORED'
bzr: ERROR: File exists: '/srv/bazaar.launchpad.net/mirrors/00/04/e1/bc'

mithrandi@verdant:~/code/CENSORED> bzr lp-submit -m 'CENSORED'
bzr: ERROR: These branches have diverged. Use the missing command to see how.
Use the merge command to reconcile them.

I think it would be a lot more helpful if the error output described the operation that failed, that way there would be more context to interpret the error.

[Tristan Seligmann (mithrandi)]

To clarify, this bug report is about the way errors are reported from lp-propose-merge in general, not either of the specific error messages I gave; those are intended just as a demonstration of how hard it is to interpret the error output.