cmd/go: clarify errors for pseudo-version commit length
These error messages contain the expected shortened revision in braces,
but don't explicitly tell the user that this is the expected one.
Just unified it with the "does not match version-control timestamp" error which does the same...
Change-Id: I8e07df7bd776fd1b39c4c90c4788cb3d626ea00b
GitHub-Last-Rev:
d14681ad086790f4e43c312c8a328d588acc6ec4
GitHub-Pull-Request: golang/go#42578
Reviewed-on: https://go-review.googlesource.com/c/go/+/269877
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>