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

首先定位 AWS Encryption Message blob 的版本和类型验证路径，并阅读当前如何构造验证错误。使用所述的字节值区分原始输入和 base64 编码输入，然后验证生成的错误能够指出可能的编码方式，同时不改变有效 blob 的处理方式。

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