devtools/drop_test_dbs.sh: continue on failure
devtools/drop_test_dbs.sh: continue on failure

Sometimes some test DBs don't exist. Change the script so it continues
in that case.

Change-Id: I1e09632cdb47bb768cfd108d537c8b053b063ebe
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/323853
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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