Send a 404 if the project files have gone away when running synctex.

This is semantically nicer than the 500 response which used to be
produced in these circumstances.

app/coffee/Errors.coffee

@@ -0,0 +1,10 @@
+NotFoundError = (message) ->
+	error = new Error(message)
+	error.name = "NotFoundError"
+	error.__proto__ = NotFoundError.prototype
+	return error
+NotFoundError.prototype.__proto__ = Error.prototype
+
+
+module.exports = Errors =
+	NotFoundError: NotFoundError