Unify the error formatting

[RiccardoM]

Context

Currently when we return the errors from the ValidateBasic or Validate methods, we have some conflicts in the formatting of the strings: some of them are capitalized while others are not. Also, sometimes we return sdkerrors.Wrap while some others we return fmt.ErrorF.

Proposal

I propose to unify the returned errors, registering custom errors where needed (using sdkerrors.Register), returning always sdkerrors.Wrap and de-capitalizing all the error strings, as suggested in the official Go documentation.