webadmin: fix error dialog padding

Commit 2e5c0cd0 incorrectly used margin in the error dialog instead
of padding. Fixed.

Change-Id: I29c94ed143b568efb1832f129b43fd9fc420b4ae
Bug-Url: https://bugzilla.redhat.com/1127333
Signed-off-by: Greg Sheremeta <gshereme@redhat.com>