Determine cause of failure from errno

[tecywiz121]

Description

Closes #8 by checking errno (via std::io::Error::last_os_error()) instead of checking the return value of flock directly.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)