encode - improve error message when there is a typo in the TransactionType @ripple-binary-codec

[khancode]

When trying to sign a transaction, if the TransactionType is invalid, the error message is cryptic and doesn't make it easy for developers to identify what the problem is.

Original issue: https://github.com/ripple/ripple-binary-codec/issues/126

[sublimator]

maybe message could included the levenshtein (sic) difference two

[JST5000]

This appears to be resolved (Although it doesn't include the levenshtein difference)