Base64 error messaging in version and type verification

[seebees]

When the version and type elements of the AWS Encryption Message format are verified,
the error message should be customize if the version and type match the base64 value.
This is convent sugar for a common problem.

Valid blobs will have the first 2 bytes in HEX will be: 01, 80, and in DEC 01, 128
Valid blobs that have not been properly base64 decoded
the values in HEX will be 41, 59, and in DEC 65, 89

aws/aws-encryption-sdk-javascript#217

[mattsb42-aws]

This will be mostly addressed by aws/aws-encryption-sdk-python#192.
What we might want to do here specifically is surface the error to the caller in a special way.