cmd/go: add a '-e' flag to 'mod tidy' and 'mod vendor'

cmd/go: add a '-e' flag to 'mod tidy' and 'mod vendor'

This flag, like the -e flag to 'go list', instructs the command to
make a best effort to continue in spite of errors for specific packages.

Fixes #26603

Change-Id: I5ee2f50c71870ae8ef3f9b3e5b045474adcca525
Reviewed-on: https://go-review.googlesource.com/c/go/+/255960
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>