refactor: improve error handling

[dwmkerr]

Might be easier to just do:

console.error('An error occurred labelling the icon: ${err}');

Another improvement would be to perhaps standardise the error logging a little, to make sure we print in colour, using chalk, and perhaps even only show the internal error message if a verbose flag like -v is set - but this can happen later.

It also means we could potentially simplify things with something like:

function errorAndExit(errorMessage, internalError, verbose, exitCode) {
   console.error(`${chalk.red('error')}: ${errorMessage}`);
   if (verbose) {
     console.error(`${chalk.red('internal error')}: ${(internalError || '').toString()}`);
   }
   process.exit(exitCode);
}

Also really I should document in the README what the error codes are and actually use separate error codes for separate conditions, but that's an improvement for another time.

Originally posted by @dwmkerr in #97