Clean Launchpad repositories when remote-build errors are raised

[dilyn-corner]

What needs to get done

Currently, if a remote-build fails due to some error, the repository will be left on Launchpad.

A specific error where this happens is when an HTTP 500 error occurs (see #4478 for a reproducible case). A specific error where this does not happen is when the snap fails to build for some reason (a legitimate build failure). In these instances, the repository is deleted in Launchpad.

I would suggest that either

• the behavior is switched (when a non-build issue is encountered, delete the repository; when a build issue is encountered, leave the repository), OR
• that the behavior is mirrored -- the repository is deleted from Launchpad in all instances where a failure occurs.

Why it needs to get done

It is probably unlikely to be useful to have an ever-growing collection of repositories on Launchpad accounts, especially repositories which users probably don't actually care about except insofar as it allows them to get a snap package back from Launchpad.