Rename EError to EAlert to match general use better

The EError mechanism is used both for error dialogs as well as basic alerts or
user prompts, so we should give it a more general name which matches this use.

This patch also cleans up a few includes of e-alert.h (formerly e-error.h) that
were not actually being used.

https://bugzilla.gnome.org/show_bug.cgi?id=602963

plugins/email-custom-header/email-custom-header.c

@@ -27,7 +27,6 @@
 #include <string.h>
 #include <glib/gi18n.h>
 #include <gconf/gconf-client.h>
-#include <e-util/e-error.h>
 #include "mail/em-utils.h"
 #include "mail/em-event.h"
 #include "composer/e-msg-composer.h"