ocsp: set GNUTLS_CERT_INVALID if OCSP response indicates revocation

This makes the OCSP based certificate verification adhere to the convention used throughout the library: "The 'GNUTLS_CERT_INVALID' flag is always set on a verification error and more detailed flags will also be set when appropriate."

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)
• CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code