internal/frontend: return 400 on non-UTF8 queries
internal/frontend: return 400 on non-UTF8 queries

Change-Id: Ie746ed2fe8f013ae0055c1fe26890301f855fc20
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/298990
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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