[dt-rfc] Non existing draft or RFC raises attribute error

[kesara]

Describe the issue

In dt-rfc branch, the API call /api/rfcdiff-latest-json/ for a non-existing draft/RFC generates an attribute error.

Example:

• https://dt-rfc.dev.ietf.org/api/rfcdiff-latest-json/rfc9999
• https://dt-rfc.dev.ietf.org/api/rfcdiff-latest-json/draft-ftie-foo-bar-00

Code of Conduct

• I agree to follow the IETF's Code of Conduct

[rjsparks]

addressed by #6681