Issue 2389:  Self signed certificate expiry causes silent send/receive failure

This is rebase of <a href="https://github.com/k9mail/k-9/pull/154">pull request 154</a> (which I will close) on the current master so that it again applies cleanly.

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Presently, k9mail silently fails when there is a certificate problem (such as when a locally stored certificate has expired).  As a result, the user may go for some time without knowing that their email isn't being fetched, or else the user may recognize that there is a problem but not know its cause.

With this patch, a CertPathValidatorException or CertificateException will create a "Certificate error:  Check your server settings" notification in the status bar.  When the user clicks on the notification, they are taken to the appropriate server settings screen where they can review their settings and can accept a different server certificate.

Link:  <a href="https://code.google.com/p/k9mail/issues/detail?id=2389">Issue 2389</a>