Referencing an undefined authentication via `.use` silently sends request unauthenticated

[mcruzdev]

When a task references an authentication by name (e.g. authentication.use: sampleDigest) and that name is not defined in the use.authentications block, the request is silently sent without
authentication instead of failing.

The fix should validate at build time that the referenced authentication name exists in use.authentications and throw an error if it doesn't.