Fix info displayed after failed decryption or verification

This ensures that the error message and the hint for missing integrity
protection (MDC) is shown. Moreover, a few more bits of information are
shown even if decryption (or verification) failed.

• Show error message for missing MDC. Previously, the generic error

message was shown because missing MDC is treated as failed decryption by
gpg so that the else-branch for m_decryptionResult.isLegacyCipherNoMDC()
was never reached.

• Show custom error message (if set). That wasn't shown for the same

reason as the missing integrity protection message.

• Show the hint for missing integrity protection. This wasn't shown

because on error the function exited early.

• Show the original filename also on error.
• Show the recipients also on error.

• GnuPG-bug-id: T8124