Better error reporting for ModelArrayRef

[timspainNERSC]

If a ModelArrayRef is missing, or is being accessed as read-write when it was shared as read-only, then an SIGSEGV occurs. This is uncatchable, and a better way of handling these errors would be valuable. As with issue #240, this could be error checking that is only active in non-production builds of the model to avoid performance penalties from excessive checking.