Make the `get_source` statement return the line numbers of top-level functions

The motivation here is for rich user experience in the web console. When a function is selected by the user via the dropdown menu, we scroll the corresponding OrlyScript to the selected function. It makes it easier for users to see the orlyscript corresponding to the function they selected. I'll also be adding a more comprehensive comments above the functions so that it can serve as the description of what the function is doing.
I don't think throwing here is going to produce the end-user behavior you want. I'd just continue, but return without the line numbers. That the source has syntax errors doesn't mean we shouldn't return it.