Decryption error reasons that should be handled

[martgil]

Description:
the ios version currently throws a generic alert box when the message can't be decrypted that says:
"Could not open message".

There are many reasons that cause the decryption error of an encrypted message needs to be handled.
Here are some of them:

1. Current key cannot open the message - often happens when the user is using the wrong private key or the sender uses an old public key to encrypt the message.

2. Missing MDC integrity checks

3. Modified MDC hash.

4. Message with the wrong checksum.

5. Unexpected end-of-steam (partially/incomplete PGP message).

6. Check if for a bogus PGP message that starts with `-----BEGIN PGP MESSAGE -----`.

Reference:
can be found under the "decrypt error" label
https://gpgtools.tenderapp.com/kb/faq/modification-detection-code-mdc-errors

[tomholub]

@martgil please retest. Does it look better now?

[martgil]

@tomholub, I have retested the latest master branch and there is only one that is not working properly at the moment.

This message has no description at all, where it should be flagged as NO_MDC - Missing integrity checks

Other than that, we can confirm that the decrypt errors are defined respectively.

MDC hash mismatch error

Key mismatch error

Invalid pgp message starting with -----BEGIN PGP MESSAGE-----

Unexpected end of stream error

wrong checksum error

[martgil]

Hi @tomholub,

I just noticed that the first key mismatch reported above was actually an error for a public key mismatch (I think)

In case you are wondering, here is what it looks like for mismatch (private key) which might be related to #1140