update decrypt error tests

[tomholub]

This will need appium tests added and updated:

• update test of subject "encrypted - MDC hash mismatch - modification detected - should fail" to check for the error message
• update test of subject "not integrity protected - should show a warning and not decrypt automatically" to check for the error message
• add another email to verify: "key mismatch unexpectedly produces a modal" where you should now be able to successfully view email body

Originally posted by @tomholub in #1219 (comment)

[fcvakintos]

@tomholub hey, i have a question
In "key mismatch unexpectedly produces a modal" email we have 2 attachments, should i check all or only the first is enough?
if we need to check all i need to update check attachments method

[tomholub]

You don't need to check the attachments in that email. We'll have a specific test for that later, after #1233 . Just test the message body is present.