objectpool: Capture fsck stderr when fetching

We have observed the git fsck process run in rescueDanglingObjects failing regularly. Sometimes this is because the process is taking longer than 75 seconds and hitting its deadline, but many times we're getting a return code. This tells us what went wrong generally, e.g. a problem with the commit graph, refs, etc., but not the precise issue with the repo.

To help understand what is causing these processes to fail, capture git fsck's stderr and include it with the error message.

Related to #4758 (closed)