url: error on paths missing leading slash for JSON APIs

[bramhaghosh]

When using the url.Filter, when the path is malformed (does not have a leading slash), Martian silently ignores it.

[admtnnr]

If we have to handle this I would rather it return an error than silently change user input. What about the other parts of the URL? If we want this validation behavior it should probably extend to any/all modifiers that deal with URLs.