Exit with timeout instead of returning an atom

Every function that called `rexi:stream/1` had to check the return value
for timeout and would then call `erlang:exit/1` if that atom were
returned. Rather than force every function to make this check this just
calls exit in `rexi:stream/1`. Its possible to catch this if a process
ever happens to require it.

BugzId: 22729
diff --git a/src/rexi.erl b/src/rexi.erl
index 75bc9bf..20f582b 100644
--- a/src/rexi.erl
+++ b/src/rexi.erl
@@ -181,7 +181,7 @@
             erlang:send(Caller, {rexi, Ref, self(), Msg}),
             ok
     catch throw:timeout ->
-        timeout
+        exit(timeout)
     end.
 
 %% @equiv stream_ack(Client, 1)