Exit command with a non-zero exit code if the externs contain an error

[realityforge]

It should be noted that this is step 1 for achieving #28

[jDramaix]

In the last commits, I've introduced a Problems class I would prefer to add a reportError() to this class and change the implementation of hasErrors to return true if an error has been reported. When reportError returns true, the generator exits with a non-zero exit code

[realityforge]

Seems reasonable. I will reimplement it that way the next time I get back to this