Check if message has structure of packed message before trying to unpack

[TimoGlastra]

Some agents return errors as the HTTP response when something went wrong. We try to unpack all received messages which can result in weird errors:

 ERROR  ERROR: error while unpacking message {
  "error": {},
  "packedMessage": {
    "error": "Unhandled exception: Couldn't locate a message handler for type https://didcomm.org/issue-credential/1.0/ack",
    "errorType": "AriesFrameworkException"
  },
  "errorMessage": "Error unpacking message"
}

We should try to see if the received message has the structure of a packed message (correct JWE keys) before trying to unpack. We could throw a dedicated error if the structure is not a JWE and notfiy the framework user we received an error message from the other agent

Relevant HTTP code: https://github.com/hyperledger/aries-framework-javascript/blob/main/packages/core/src/transport/HttpOutboundTransport.ts#L78-L81

Encrypted message structure: https://github.com/hyperledger/aries-framework-javascript/blob/main/packages/core/src/types.ts#L15

function A(message: any): message is EncryptedMessage {
  return message.protected
}

Example encrypted message:

    {
      protected: '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',
      iv: 'KNezOOt7JJtuU2q1',
      ciphertext: 'mwRMpVg9wkF4rIZcBeWLcc0fWhs=',
      tag: '0yW0Lx8-vWevj3if91R06g=='
    }