This change starts observers as soon as possible, and wraps the entire
block that can cause an exception in try..finally. This ensures that we
stop threads are stopped.

If we notice the same behavior, it could be worth to put a cap on number
of executions per observer, and shut the thread down once reached.

R=apolito@google.com

Bug: 1255228
Change-Id: I1f165267460da02b3bbba39022c262e6c29fe86b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3213451Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>