Better error messages about plasma-mvp-sidechain

[AdityaSripal]

Already done on my PR lol. Sends "deposit not final" along with how many blocks must pass before it is finalized. Of course from client side this is just an approximation since they may see a different chain than validator, but still useful info

[colin-axner]

When spending a deposit if presumed finality has not passed, an error message of "deposit not final" should be returned instead of "deposit does not exist"

[colin-axner]

When trying to spend a tx signing with the wrong key I get this error:

{"codespace":"handlers","code":1,"message":"input, [(0.0.0.0) Tu\ufffd\ufffd\u0001\ufffd;\ufffd\ufffd$\ufffduN(h۸)\ufffd:], does not exist by owner %!!(MISSING)x(MISSING)"}

Should replicate this error msg later, and adjust where it is being returned to be more clear

[AdityaSripal]

Strange, why would it break on 0.0.0.0 not existing (Im assuming that this is second input, and u mis-signed on first input)? Looks like it's just a bad error msg overall and we need an "incorrect signature" error msg to be returned

[colin-axner]

I think what might have occurred is since I signed with the wrong key, the utxo returned a nil position because it didn't exist in the mapping, causing the error message above