[Runtime] Return a "NOT FOUND" message if the resource is invalid when using application scheme.

When using application scheme (app://), if loading a resource which does
not belong to an application, if should return the error message
directly, this patch will do this if the application can not found.

BUG=XWALK-1762
Merge pull request #2053 from takethathe/app_scheme

[Runtime] Return a "NOT FOUND" message if the resource is invalid when using application scheme.