show more detailed error message if a key cannot be imported

[r10s]

When trying to decrypt a setup message and the decryption succeeds, we should check the decrypted content and show more appropriate error messages if the payload does not have the expected format. see https://github.com/deltachat/deltachat-android/issues/279

the same should apply for password-protected-keys and for manual imports of keys.

[kolbma]

And it is very strange that you have to specify the directory for import and not a file. And there is no information which files the importer is looking for.