cmd/frontend: exit if loading fails in local mode
cmd/frontend: exit if loading fails in local mode

As there's no way to load modules in local mode while the server is
running, exit if all loading fails.

Change-Id: Ia4d46ad8c3937a029fb7bfb0552b774d61efd244
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/269259
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

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