Improve error handling

[humphd]

If there's an error (e.g., processing a file), you should give a useful error message to the user, and decide whether or not the program can continue. If the error puts things in a state that can't be recovered (e.g., working on a file that doesn't exist), you'd be better off to log the error and then exit the program: process.exit(1) using a non-zero exit code.