Add a check for already-running backup

[krair]

Sometimes there's a backup still running from the previous day/attempt. Starting another run does not work, and only adds more PIDs that need to be killed later.

Option 1:

• If previous run is still running, cancel new run to let the previous finish

Option 2:

• Kill previous run, unlock repo, start new run

Caveats:

• Is the previous run still actually running, or is has it failed but is in a "stuck" state? How can I retrieve this information to automatically chose from the above 2 options?
• Should I add a simple feature flag to the script to cancel/kill while figuring out how to answer the prior question?