Add a watch() variant by string name instead of scoped key

This multiplexes by looking up all scoped keys for the name
and watching all of them. If there are no keys, it is
guaranteed to return an error.

Stubbed out for now in the actor API.
Add "raw" flavors of watch(taskName)

Activator wants the TaskResult and not the Try
Convert "no results of key lookup" to an error in by-name watch
Merge pull request #234 from sbt/wip/havocp-watch-key-name

Add a watch() variant by string name instead of scoped key