SYM/FTPT download broken behavior for missing models

[ryansturmer]

The Footprint Ref/Library Ref fields which allow downloading of the SVN models are broken if the models don't exist. Link should route to a friendly error message, or be grayed out, rather than ending at the exception page.

To reproduce:
Create a new part, and give it a bogus footprint ref and/or library ref
Go to that part in the parts table and click on the link to download the bogus file
Profit.