[CBBS] Poor error message for failed task runs

What's the issue?
When a 'cbbackupmgr' task fails, we use the error from 'cmd.Wait()' as the reason for the failure, in reality, this will usually only be the exit status of the command which isn't useful for debugging (especially if we don't have access to the logs).

We should "scrape" the last line of output from 'cbbackupmgr' which should contain the full error message which is usually printed to stdout.

We need to be careful that this doesn't contain any user data, if that's a possibility, we can always just log the output in user-tags, and leave the status code error (if we have access to the unredacted logs we'll have plenty of information to debug).

https://github.com/couchbase/cbbs/blob/ee8d00aa8652283078c7627186b79aaacb89248c/grpc/task_runner.go#L247