Track and notify unfinalized/uncaught promise chains

[orthecreedence]

Take this example:

(chain 4
  (:then (x) (+ x 'test))
  (:then (x) (save-to-db x))

The promise chain is broken but the error isn't caught (unless attach is called on the chain call). If we can use trivial-garbage to detect these dangling chains and run some sort of callback, it would aid in the debugging of improperly handled chains.