dep_graph: avoid panicking in thread when channel closed
dep_graph: avoid panicking in thread when channel closed

On my system, when the processor is already loaded, and I try to
run the test suite, e.g. compile-fail/dep-graph-assoc-type-trans.rs
fails because of undecodable JSON.

Running the compiler manually, I can see that the dep graph thread
panics (and puts non-JSON on stderr) while `send`ing on `swap_out`,
presumably because the other end has already quit.  I think that in
this case, we can just gracefully exit the thread.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust