cmd/go/internal/get: propagate parse errors in parseMetaGoImports

The signature of parseMetaGoImports implies that it can return an error,
but it has not done so since CL 119675. Restore the missing error check,
and remove the named return-values to avoid reintroducing this bug in the
future.

Updates #30748
Updates #21291

Change-Id: Iab19ade5b1c23c282f3c385a55ed277465526515
Reviewed-on: https://go-review.googlesource.com/c/go/+/189778


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>