Base64 error messaging in version and type verification

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`

https://github.com/awslabs/aws-encryption-sdk-javascript/pull/217

Start at the version and type verification entry point for AWS Encryption Message parsing, and compare the stated valid byte values with the base64-encoded values. Review the behavior discussed in pull request #217. Done means a distinct, useful error is produced when the input appears not to have been base64-decoded, while normal verification remains unchanged.

Written by the indexing model from the issue text.