Fix #470

This is where I'm up to with this. Trying to use a #load or #r after the start of code now throws an exception which results in a message like this:

ERROR: Encountered #load directive after the start of code. Please move this directive to the beginning of the file.

Previously, these directives were being ignored in DirectiveLineProcessor's ProcessLine method and I've implemented the new error at this level. Is this correct? Or will some directives inheriting from this want to ignore but not throw an error.