cmd/go: fail 'go work' subcommands with a more helpful error if no go.work file exists

Otherwise, the failure mode for these subcommands refers to an empty file path:
	go: open : no such file or directory

Fixes #50964

Change-Id: I8776431a294d2b2246d7d147b6059054f31bc255
Reviewed-on: https://go-review.googlesource.com/c/go/+/382246


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>