[inspector][test] Exit properly on internal errors

Calling {Exit} does not stop execution in the current thread (or
process), it just tells the two task runners to stop. Hence following
code would still be executed.
This CL replaces the calls to {Exit} to {FATAL} for internal errors
where we want to stop immediately.

R=szuend@chromium.org

Bug: chromium:1142437
Change-Id: I70c1c39da28a3cd040214d46eddf61f05a12bce7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2519568Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70984}