CLI: exit process with correct error code

[lo1tuma]

The CLI tool just logs errors and warnings to stderr and exits the process as normal with exit code 0.

If you use some CI tool like jenkins, your build will be marked as successful.
The output on jenkins looks like this:

> aglio -i api.md -o build/api/index.html
>> Line 101: undefined symbol 'foo' (error code 3)
Finished: SUCCESS

[danielgtaylor]

Thanks, I'll see about fixing this to output a non-zero exit code.