CL 513756 moved the usage of modGo after it was set to its proper value,
so the error message text always listed the version of go as
"unspecified". Fix the error message in the case where the version was
set in the go.mod, and provide an error message where it wasn't.
Fixes #67587
Change-Id: I763f6be7ee811da32fcb7e785682fd6f48145981
Reviewed-on: https://go-review.googlesource.com/c/go/+/588075 Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>