This simplifies the lengthy main method by extracting some code and
by replacing the scattered returns with exceptions.

We introduce two exceptions for early bail-out. This enables helper
methods on multiple layers. The early bail-out on time-out is
moved to the point where it is detected.

Previously on timeout and crash we also printed out the step number.
Clusterfuzz doesn't parse this, it was only for statistical purposes,
and the latest version of the experimental workbench only parses
crashes and timeouts, not the step in which they happened. Hence,
this CL removes those step numbers.

Except the change described in the last paragraph, this CL doesn't
intend to change behavior.

No-Try: true
Bug: chromium:1100114
Change-Id: Ie8c18f183e4fc538577f3eb49aaf6df1acd1e4e1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2270547Reviewed-by: Liviu Rau <liviurau@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68576}