internal/frontend: return 404 when package does not exist
internal/frontend: return 404 when package does not exist

Currently, a 400 is returned when a package is not found. A 404 is now
returned instead.

Fixes b/136463270

Change-Id: I61e4060b3aafcd6d89d6aff588f875aed53e64a3
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/503136
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite