[dev.fuzz] testing: fix internal error exit condition for fuzz workers

[dev.fuzz] testing: fix internal error exit condition for fuzz workers

A fuzz worker process should exit with fuzzWorkerExitCode (70) if it
encounters an internal error.

This should generally only happen due to an I/O error on the worker
side. I can't think of a way to write a realistic test case for
this. test_fuzz_io_error.txt triggers an I/O error on the coordinator
side, which is similar.

Change-Id: I40d2e9aaf422e5ed925debcf7a152c252c3a6984
Reviewed-on: https://go-review.googlesource.com/c/go/+/349993
Trust: Jay Conrod <jayconrod@google.com>
Trust: Katie Hockman <katie@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>