improve docs and Promise impl
improve docs and Promise impl

- scaladoc the exceptions thrown by Await.* and Awaitable.*
- move intercept[Exception] into partest’s TestUtil object
- improve Promise.tryAwait implementation following Viktor’s comments
  and make use of Deadline to avoid calling System.nanoTime too often

scala/scala-partest,lrytz/scala-partest,scala/scala-partest,som-snytt/scala-partest,SethTisue/scala-partest