handle io errors with fincore more gracefully

- ensure cli traps IOError as well as OSError, to caputre
  issues opening or reading a path
- reraise exceptions with a simpler error message better
  indicating what went wrong
- added test case for a failing fincore command
- Resolves issue #72
flake8 cleanup

- errno import was masking a local variable
Merge branch 'feature/fincore_handle_invalid_paths' into develop