Add status code error when throw SwiftyRSAError

[xandros6]

When a SwiftyRSAError is throw the message with the exception not include the error code of SecKeyDecrypt, so debugging is difficult.

[ldiqual]

@xandros6 Thanks a lot for opening an issue! I converted SwiftyRSAError from a class to an enum so it can be introspected. Check out the master branch with cocoapods and let me know if it works for you!