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 customized 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 will be: 0x01, 0x80.

If the blob is base64-encoded, the values will be 0x41, 0x59.

aws/aws-encryption-sdk-javascript#217