don't attempt to compile directories
using pylons, if you attempt to render a non-existent template, the engine generates a TopLevelLookupException. however if you pass a filepath which refers to a directory, an IOError is raised instead. imho this is also a lookup error, not an IO error.
this should probably be checked against in line 69 of template.py.
an easy workaround is to catch the IOError -- perhaps the default in pylons (or at least the template controller) should try to catch that (making this a pylons bug and not a mako bug.) but it seems like an inconsistency.....