cmd/go/internal/generate: error if failed to find a package
Add check for package loader to print error and fail `go generate` command,
if package can not be found.
Fixes #60079
Change-Id: Ib9e730c2b69df6e5ac307c7bdfea0ee993ab6ed8
GitHub-Last-Rev:
d93332425a980d0298c74f33f1d154e0afbf5373
GitHub-Pull-Request: golang/go#60178
Reviewed-on: https://go-review.googlesource.com/c/go/+/494836
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>