Error message not concise

[patrickwieth]

So this is not a real bug, but an error message that could be more helpful:

https://github.com/tendermint/go-amino/blob/master/amino.go#L337

The error message fits well for the case in L339 but in L337 it should not inform that the prefix bytes are wrong but are missing. I thought I messed up registering the codec or something like this but actually the data was empty due to another bug.

[liamsi]

Thanks @patrickwieth!