Properly handle invalid base32 characters

[ahawker]

As of today, it is possible to input non-base32 characters, uU for example, into any of the api calls.

Doing this will cause the library to fail silently and perform an incorrect base32 decode on the string.

The API should provide a feedback mechanism that informs the caller of the bad input. The implementation of that feedback is still TBD (separate API call vs. exception vs. ??).

Considerations:

• Performance of this computation for every decode call?
• Double-penality for callers that have already made this guarantee?
• Separate API call to validate? Is there use-cases for this outside of normal hot path?