[settings] Handle errors in fetchf_uristring()

[settings] Handle errors in fetchf_uristring()

fetchf_uristring() was failing to handle error values from
fetch_setting(), resulting in its attempting to allocate extremely
large temporary buffers on the stack (and so overrunning the stack and
locking up the machine).

Problem reported by Shao Miller <Shao.Miller@yrdsb.edu.on.ca>.