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 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`.

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

Start by locating the version and type verification path for AWS Encryption Message blobs and read how validation errors are currently constructed. Use the stated byte values to distinguish raw from base64-encoded input, then verify that the resulting error identifies the likely encoding without changing valid-blob handling.

Rédigé par le modèle d'indexation à partir du texte de l'issue.