Improve type ergonomics and add Error type

[ebkalderon]

Added

• Add common Error type to use throughout the library.

Changed

• Return PathBuf from get_capture() instead of String.
• Return &Path from get_{log,capture}_file_path_template() instead of &str.
• Accept Into<PathBuf> or Into<String> in some places to avoid extra allocation if possible.
• Document if path argument is None in docs for set_capture_file_comments().
• Switch from lazy_static to once_cell.

These changes were long overdue, and this crate hasn't seen any love in years. It's time to change that!