Handle the case where current_exe doesn't exist

We currently perform several checks on `current_exe`. However, in special cases it's possible for `current_exe` to point to a path which no longer exists - for example, if something up the stack renamed the EXE while it's running. We should handle this gracefully.

Locate the existing checks on `current_exe` and read the surrounding updater flow. Exercise the case where the executable path no longer exists, then verify that the updater handles it gracefully without an unexpected failure.

Written by the indexing model from the issue text.