Improve command line interface error feedback

This issue could be solved by a junior Browsh developer. I hope it will encourage someone to get involved.

Currently when you simply start the Browsh binary without command line arguments, you get something like this:

./browsh_1.4.12_linux_amd64 exit status 1

The user doesn't know for what reason an error occurred.

Possible ways to improve this are to:

• display the usage/help output, like --help
• and/or to display an error message such as "Firefox binary not found and no path given. Use --firefox.path"