Fixes #7026/BZ1128471: fix error on updating errata date range filter.

There was an error when updating an errata data range filter.  This
commit fixes the error as well as displays error messages on the form
in the event of a legitimate error.

http://projects.theforeman.org/issues/7026
https://bugzilla.redhat.com/show_bug.cgi?id=1128471