PatientServiceImpl.saveReasonForExit needs to provide a change message when updating the reason for exit obs

The saveReasonForExit obs needs to specify a change message when updating the reason for exit obs. Otherwise, you get the following error:

An Internal Error has Occurred
org.openmrs.api.APIException

ChangeMessage is required when updating an obs in the database

This occurs, for example, if you try to call "exitFromCare" on the same patient twice.