Make valueChanged stream set value to null on task failure

Previously we were probably trying to use an exception value
as the value on task failure or something.

Adjust discoveredMainClasses handler a little to handle null
values.
Track sbt-echo inspect support tasks

inspectSupported and whyInspectIsNotSupported observables
are probably the most interesting ones.
Merge remote-tracking branch 'origin/wip/sbt-server' into wip/havocp-echo-supported

Conflicts:
	ui/app/assets/services/sbt/tasks.js
Merge pull request #761 from typesafehub/wip/havocp-echo-supported

Track inspect support tasks