[#2013] Fix for paster serve restart bug

paster's restart-monitor gets a list of files by iterating over sys.modules,
which appears to include Toolkit. It calls __file__ and that causes the exception.
However paster catches an AttributeError fine and continues, whereas a plain
Exception causes the whole restart-monitor thread to crash. AttributeError also
makes more sense for a failed class attribute access.
Merge pull request #2014 from ckan/2013-restart

[#2013] Fix for paster serve restart bug