Add missing exc argument to PathError

[althonos]

Type of changes

• Bug fix

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @PyFilesystem/maintainers may be pedantic in the code review.

Description

fs.errors.PathError was missing an exc argument, so it couldn't be used in fs.error_tools.convert_os_errors. This prevented the conversion of OSError(errno.ENAMETOOLONG) errors to PathError. Closes #453.

[coveralls]

Coverage increased (+0.0009%) to 95.363% when pulling eebb4a2 on fix-patherror into 60b0a63 on master.