Add readme note about globbing Gemfile for bundleExec

In order for bundle exec to work you have to have the gemfile in the directory
you're running the command from. Kind of weird in this context but eh.
Add more intellegent error handling.

For posterity:

- Sass fatal errors (bad syntax, missing files) logs to stdout, exit code 1
- Bundle missing gemfile, logs to stdout, exit code 10
- Bundle missing gem entirely, logs to stdout, code 127

- Sass debug and warn directives, logs to stderr, code 0
- Sass deprecation warnings, logs to stderr, code 0
- Bundle "Could not find gem 'sass (= xxx) ruby" plus huge stack trace, logs to stderr, code 1

- Missing executables (no Sass, no Bundler, no Ruby) causes node error,
	logs 'execvp(): No such file or directory' to stderr,
  logs 'Error: spawn ENOENT' on error
  code -1
Refactor special error handling to be less ugly