Error handling

[chp2001]

Adds:

• Additional error handling/context for cli.rs, routing.rs, and main.rs.
• Added basic in-file unit tests for cli.rs and config.rs.
  • More thorough testing of arguments to the program will likely require extensive refactoring.
• Added testing suite for main.rs that verifies the test case works as expected.

Modifies:

• Added static OUTPUT_TYPE variable to main.rs along with a match statement to suppress rustc's warnings about unused enum elements. This should serve as a placeholder until the relevant features are added.
• Added unused _config_dir variable to the main function in `main.rs' to suppress the warnings about the CLI Config struct config_dir member going unused. Given that using the configuration data is on the roadmap, this was preferable to outright removal.
• Slight refactoring in routing.rs to suppress unused variable warnings.
• Applied rust-fmt to full project to improve code consistency and format.

PR likely is not complete, as there are probably additional areas that need error handling added.