[master] errors during cleanup mask underlying errors (eg TooManyConcurrentRequests)

[Martin Pool]

There is a general problem in bzr that errors raised during finalization can mask other more important or illuminating exceptions raised earlier on. See for example <https://bugs.edge.launchpad.net/bzr/+bug/423015>. Also, the errors during cleanup are often (but not always) caused by knock-on effects of the earlier problem, such as the smart medium being broken.

We need to have some way to make the earlier error visible.

Constraints:

 * Errors during cleanup should not be lost: if they indicate something has really gone that should be visible to the user, to tests, and to API users. (Consider a bug causing an UnboundLocalError during cleanup.)

 * Code that runs a cleanup through a try/finally block should not be overly complicated.

I propose to make all code likely to be called at cleanup (unlock, abort_write_group, etc) go through a common higher-order-function that can define policy for errors within them, then add a -Dcleanup that will turn those errors into warnings, thereby allowing the earlier error to continue. This shouldn't make anything worse, and it will give a way to at least see what the earlier error was without needing to change the source.

[Robert Collins (lifeless)]

Sounds fine to me