Generate non-inclusion proof correctly

[vrogojin]

When requesting an aggregator to return a proof for a requestId that have never been submitted, rather than returning 404 error, return valid non-inclusion proof

[b3y0urs3lf]

Tested using aggregator_tag=431e6390cb5a7ad921a64868974c79f4cd6d23c2 using https://aggregator-test.mainnet.alphabill.org and locally running CLI on commit ac98e50bb4e5223824b793da82f7eb86a3ed161b.

After implemented amendments on aggregator side I see that all previously created endpoints still working (user cna submit commitments and get proofs and also check block info).

Also via CLI user can perform GET proof action

and if requestID wont be found, then response would contain PATH_NOT_INCLUDED

Moving task to done.