executor: fail early on reattach if listener is not executor

[mismithhisler]

Description

Fixes a bug reported by Windows users where Nomad was killing random processes on reboot. This was due to the reattach config containing a PID and RPC address that was now occupied by other processes/listeners. When Nomad would attempt to make an RPC, it would fail, and kill the process. With this change, we fail early on reattach, and do not kill the process.

Testing & Reproduction steps

Reproduction involves simulating this scenario via overriding the reattach PID when starting a task to a known PID (another bash session), rebooting Nomad, killing the executor process, and listening on the reattach address.

Links

Fixes GH #23969

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[tgross]

LGTM! Nice work on this!

[jrasell]

LGTM, thanks @mismithhisler!