internal/frontend: 404 on unknown routes instead of showing home page
internal/frontend: 404 on unknown routes instead of showing home page

Previously, typing any unknown path (i.e. /aksldnas) would return the
homepage. Now it returns a 404.

Change-Id: I2d8d73f19cb3b85ad216f4493b116541ab2eeb39
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/495937
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

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