Add a message to the UserError raised when trying to do a Dir lookup with create=False.  Should not normally happen, but it did to me, and it is better to have some explanatory message when it happens rather than failing silently.
Add a message to the UserError raised when trying to do a Dir lookup with create=False.  Should not normally happen, but it did to me, and it is better to have some explanatory message when it happens rather than failing silently.

git-svn-id: 7892167f69f80ee5d3024affce49f20c74bcb41d@4570 fdb21ef1-2011-0410-befe-b5e4ea1792b1

azverkan/scons,azverkan/scons,azverkan/scons,azverkan/scons,azverkan/scons