Exit with an error if scanf fails

[larsks]

Exit if scanf returns an error, rather than going into an infinite loop if we are unable to read from stdin.

Closes #58

[zyedidia]

I think this is reasonable but it should only exit if the error is io.EOF. If it's a different error, like a failure to parse an integer, that should be reported to the user without exiting.

[larsks]

I've updated the code to only exit when we hit an EOF error. The existing code correctly produces warnings for other sorts of invalid input.