ephy-certificate-dialog: reword bad identity error message

This wording should be a bit less confusing.
ephy-certificate-dialog: remove old code

We do not use the action area now that we have header bars
ephy-certificate-dialog: fix position of lock icon

If the cert is untrusted then the lock icon displays parallel to the
first row of text; it needs to be centered between them.
ephy-certificate-dialog: display window title

The blank header bar feels quite odd.