LP#1879983: Predefined ILS Events to alert the UI

LP#1879983: Predefined ILS Events to alert the UI

This commit defines events that might be sent to UIs in the case of
exceptional circumstances:

* CURBSIDE_NOT_ALLOWED (i.e., if the circ.curbside library setting is
  not enabled for a given pickup library.)
* CURBSIDE_MAX_FOR_TIME (i.e., if all of the available slots for
  a given appointment time have been scheduled.)
* CURBSIDE_EXISTS (i.e., if one is attempting to create a new
  open curbside appointment for a patron and pickup library when
  one already exists.)

Sponsored-by: PaILS
Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>
Signed-off-by: Michele Morgan <mmorgan@noblenet.org>