Improve error handling

[ebcrowder]

Fixes:

• Improves error handling by defining custom errors for more informative messaging for users.
• 3 custom errors have been defined: InputError, CSVError and Other. These errors would catch incorrect arguments, errors in CSV parsing and other situations, respectively. None of the code is consuming the Other error type currently, but it is there for future utilization if we need it.
• The InputError now prints this to stdout: Error: InputError(Custom { kind: InvalidInput, error: "Invalid argument was entered. Please try again." })

Tests:

• Build passes
• Clone this branch locally. Invoke the CLI with an incorrect argument (such as reg instead of register) and note that the error message is bit more helpful.

[ebcrowder]

Closing this as it will be implemented in an upcoming PR.