cmd/gopherbot: skip commenting on nonexistent issues
cmd/gopherbot: skip commenting on nonexistent issues

When an issue is transferred, we incorrectly continue trying to manage
the issue with Gopherbot, even though we should be able to pick up the
new issue. As a temporary fix, this change skips issues which return a
404 when trying to comment.

For golang/go#40640

Change-Id: I9280fab7a8ec4ead0d3e1cc5695d9b7eb39d6b32
Reviewed-on: https://go-review.googlesource.com/c/build/+/247409
Run-TryBot: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/build,golang/build,golang/build,golang/build