Handle errors when an invalid outpoint is requested

[afilini]

Instead of printing horrible panic messages, gracefully handle the error and show a human-readable message when a request for an invalid outpoint is sent

[isghe]

Let's solve also the:

thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: ParseFailed("String was not valid UTF8")', src/libcore/result.rs:1009:5

generated with sendtoaddress, burn and sync.

the problem looks to be at:

10: <bifrost::server::RGBServer as hyper::server::Handler>::handle
             at src/server.rs:90

https://github.com/rgb-org/bifrost/blob/938c45549bcefa761cfaac9f36d1b91628e441a6/src/server.rs#L90

[isghe]

sendtoaddress:

Could not decode the uploaded proof for `/0fc882921eee7219ea267b8a3c8f969466dedcae6344fc3511cf68862402d83b:0`: parse failed: String was not valid UTF8

burn:

Invalid outpoint in POST req for `/e579cf5fca529091e9ac5a23a1a23c7dc6ada7dee8962d4aea92713461617215:BURN`
Could not decode the uploaded proof for `/e579cf5fca529091e9ac5a23a1a23c7dc6ada7dee8962d4aea92713461617215:0`: parse failed: String was not valid UTF8

and the folder .rgb-server is empty.