[dev.fuzz] internal/fuzz: crash if there is no error output

[dev.fuzz] internal/fuzz: crash if there is no error output

Previously, the coordintor used the error string encoded by the worker
to determine whether or not a crash occurred. However, failures caused
by things like t.Fail() which have no output will have an empty error
string, so we can't rely on the error string alone to determine if
something is a crasher or not.

Change-Id: Idcf7f6b1210aa1dc4e8dab222642c87919595693
Reviewed-on: https://go-review.googlesource.com/c/go/+/298351
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>