internal/frontend: serve error on nil queue
internal/frontend: serve error on nil queue

Requesting a package or module using a default branch like "@master"
causes the frontend to enqueue a request to the backend.

For cmd/pkgsite, there is no backend, so this behavior doesn't make
sense.

Prior to this CL, cmd/pkgsite panicked in this case because the
server's queue client was nil.

Here we serve an error instead.

Fixes golang/go#54478.

Change-Id: Id6f19ddd38b68b71286b5aaba9c83e676f4aa04c
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/425034
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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