Reject invalid priority labels in SetPriority

SetPriority converts its argument to a number before storing, so a value
that was neither a number nor a configured PriorityAsString label was
silently coerced to 0. The web forms use numeric inputs, but other callers
(the REST2 API, the CLI, scrips) could still zero out a priority with a bad
value. Return an error and leave the priority unchanged instead.

Author: sunnavy

lib/RT/Ticket.pm

@@ -3379,9 +3379,10 @@ sub SetPriority {
     else {
         # Try to load a number from the string
         $number = $self->_PriorityAsNumber($priority);
+        return ( 0, $self->loc( "Invalid value for [_1]", $self->loc("Priority") ) ) unless defined $number;
     }
 
-    return $self->_Set( Field => 'Priority', Value => $number || 0 );
+    return $self->_Set( Field => 'Priority', Value => $number );
 }
 
 =head2 TimeEstimated