Verification error messages are unhelpful

When attempting to verify someone's website, I received error code 240 or something. Further inspection revealed that the `keybase.txt` file isn't there. Then I bugged him and he fixed it, and i got another error code, which didn't help, which I believe was caused by a domain mismatch (he had a subdomain in the verification message). These things should/could be explained in the error message. He fixed it so I unfortunately can't replicate the issue easily.

No files, tests, or entry points are named. Start by locating the website verification flow and the handling for error code 240, then determine how missing keybase.txt files and domain mismatches are reported. Done means these failure cases produce messages that explain the cause rather than only exposing opaque error codes.

Written by the indexing model from the issue text.