VTBackup continues working even if MySQL exited unexpectedly

[frouioui]

Description

We have observed several vtbackup processes that would continue trying to catch up from the primary after restoring the latest backup even if the mysqld process has been killed. Since the for-loop where we catch up does not have any mechanism to "give up" after a certain number of retries, we continue this indefinitely. Leading to similar logs:

Ideally we should have add a new callback when initializing the mysqld process, that would be call on error and that would cancel the vtbackup process.