Display a proper message when key parameters are invalid

[le-git-imate]

Description of issue or feature request
Upon the key evaluation, users must be notified with a proper message if any key parameter (such as wrong userId, expired key, etc.) is wrong. This issue could be fixed along with #1, #14, #22.

Current behavior
The extension does not always display proper messages if a key parameter is not valid. Currently it only checks userid and email for keys generated by the extension.

Expected behavior
All key parameters (e.g., userId, email, expiration date, key size, etc.) should be validated before adding the key into the local keyring.