Don't throw CertificateValidationException for all SSLExceptions.

An interrupted connection attempt to the server yields an SSLException
as well, like this:

E/k9      ( 6937): Caused by: javax.net.ssl.SSLHandshakeException: Connection closed by peer
E/k9      ( 6937):      at com.android.org.conscrypt.NativeCrypto.SSL_do_handshake(Native Method)
E/k9      ( 6937):      at com.android.org.conscrypt.OpenSSLSocketImpl.startHandshake(OpenSSLSocketImpl.java:302)
E/k9      ( 6937):      at com.android.org.conscrypt.OpenSSLSocketImpl.waitForHandshake(OpenSSLSocketImpl.java:598)
E/k9      ( 6937):      at com.android.org.conscrypt.OpenSSLSocketImpl.getInputStream(OpenSSLSocketImpl.java:560)
E/k9      ( 6937):      at com.fsck.k9.mail.store.ImapStore$ImapConnection.open(ImapStore.java:2459)

We don't want the user to notify of 'certificate problems' in that case.
Fix it by checking whether the SSLException was actually triggered by a
CertificateException.
I don't know Java very well, but shouldn't it be possible to just catch CertificateException here?