fix some sloppy error handling in imports
fix some sloppy error handling in imports

1. print something useful if an imported file cannot be
    accessed for some reason (i.e. permissions)

2. don't make assumptions about where the system files
    are located; use is_system_file to test whether a file
    is or isn't a system file

BBN-Q/pyqgl2,BBN-Q/pyqgl2