Search backend: fix user-facing error strings
Created by: camdencheek
When a user enters an invalid patterntype, the formatting directive %v does not actually show the string contents of the pointer, just the pointer value. These end up being user-facing errors, so they should look nice.
Test plan
Added a probably-unnecessary test for the formatting of the errors in this function, but since they are user-facing, seemed maybe worth the few lines.