This change makes ModHandle and ModTidyHandle return a ModData structure that only contains an error if something goes wrong when parsing the .mod files.
Change-Id: I89661cfefeff666bdf13cf99f4c56137e120de82
Reviewed-on: https://go-review.googlesource.com/c/tools/+/222242
Run-TryBot: Rohan Challa <rohan@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>