Fewest clicks between two wiki pages

Wiki pages link to other pages. Given a start page and a target page, return the fewest clicks needed to get from one to the other, or report that the target can't be reached. You only see links through a function that returns the outgoing links of a page, and you are asked to write a small simulator of that function too.

Follow-up: the link function can fail or time out. Make the search robust to that, and discuss retries and what answer you return when some pages could not be read.