[signingscript] Validate unsigned files conform to autograph's naming requirements

[ahal]

While working on some VPN signing, I got generic 400 Bad Request errors out of autograph. After digging through logs, Hal found that the error was:

invalid named file name: unsigned filename must match ^[-_\\.a-zA-Z0-9]{1,256}$

It would be nice if autograph could provide a better error response, but failing that it would be nice if signingscript validated the files it was going to sign so we can detect this more easily.