x/vuln: improve error message when running on invalid pattern

[julieqiu]

Related: #59607

In github.com/u-root/u-root

govulncheck pkg/strace/... returns:

~/code/u-root main ❯ govulncheck pkg/strace/...                        04:54:49 PM
govulncheck is an experimental tool. Share feedback at https://go.dev/s/govulncheck-feedback.

Using go1.20.3 and govulncheck@v0.0.0 with
vulnerability data from https://vuln.go.dev (last modified 2023-04-12 21:45:55 +0000 UTC).

Scanning your code and 0 packages across 0 dependent modules for known vulnerabilities...
No vulnerabilities found.

We should report that the pattern is invalid, instead of there being 0 packages.

[gopherbot]

Change https://go.dev/cl/532056 mentions this issue: internal/scan: return error when patterns matches no packages