CiviCase "Max instances" does not work nicely

(In discussion of CRM-14798, a few issues have come up with using max instances. Below is a comment I've copied from dgg.)

Max instances - Fatal error has been fixed. However, the behavior when trying to add an activity when max instances would be exceeded is 'ugly' and confusing:

• pop-up dialog appears and stays empty
• "Network error - unable to reach the server" error status box appears
• The expected "info" status box appears as well with user message and invitation to 'edit the existing activity'.

Seems like we should exit and put up user message w/o trying to load the pop-up form.