Update default_validator; add custom error when user output is empty

[JoelNiemela]

When the user output file is completely empty, add the error message "user output was empty"; when the user output file contains only whitespace, add the error message "user output contained no tokens".

Other changes:

• Rewrite if (...) stmt; as if (...) { stmt; }
• Explicitly assign zero to judgeans_pos and stdin_pos (line 17). Previously, they were implicitly initialized to zero (since they are global variables).
• Initialize judgeans_line and stdin_line to 1 (line 18). Previously, they were set to 1 in main(), before the while (true) loop.
• Add some whitespace to be more consistent.

[gkreitz]

Thanks, looks good!

I suggested one minor change to a message. Happy to merge this as-is too (if you prefer the current message, or if I my suggested phrasing is incorrect).