This commit:
* moves defined errors to a separate file;
* renames ErrEmptyToken to ErrNilToken;
* merges ErrZeroTTL and ErrIncorrectTTL into single ErrInvalidTTL.
This commit adds next changes to VerifyTokenSignature:
* returns ErrEmptyToken on nil token argument;
* returns ErrEmptyPublicKey on nil public key argument.