Misleading error message when failing to open file (ex: permission denied)

[jeffenstein]

When a .pyc file cannot be opened, this is the error message:

Traceback (most recent call last):
  File "/home/jeff/.local/bin/uncompyle6", line 11, in <module>
    sys.exit(main_bin())
  File "/home/jeff/.local/lib/python2.7/site-packages/uncompyle6/bin/uncompile.py", line 181, in main_bin
    **options)
  File "/home/jeff/.local/lib/python2.7/site-packages/uncompyle6/main.py", line 223, in main
    linemap_stream, do_fragments)
  File "/home/jeff/.local/lib/python2.7/site-packages/uncompyle6/main.py", line 126, in decompile_file
    source_size) = load_module(filename, code_objects)
  File "/home/jeff/.local/lib/python2.7/site-packages/xdis/load.py", line 110, in load_module
    fp.close()
UnboundLocalError: local variable 'fp' referenced before assignment

Changed to use with context handler so that the real exception is raised:

Traceback (most recent call last):
  File "/home/jeff/.local/bin/uncompyle6", line 11, in <module>
    sys.exit(main_bin())
  File "/home/jeff/.local/lib/python2.7/site-packages/uncompyle6/bin/uncompile.py", line 181, in main_bin
    **options)
  File "/home/jeff/.local/lib/python2.7/site-packages/uncompyle6/main.py", line 223, in main
    linemap_stream, do_fragments)
  File "/home/jeff/.local/lib/python2.7/site-packages/uncompyle6/main.py", line 126, in decompile_file
    source_size) = load_module(filename, code_objects)
  File "build/bdist.linux-x86_64/egg/xdis/load.py", line 105, in load_module
IOError: [Errno 13] Permission denied: 'test.pyc'

[rocky]

Sure. Thanks.

(Note that in the Python 2.4 branch we can't make this change.)