better messages in dotsession

When messages stop making sense even to the author, it's a clear sign :)
redraw debug focuses on disconnect
stack traces!
watches!
better reporting for unhandled exceptions
how can self.v be falsy if it's not None?!
a minor todo which finalizes today's development session
makes colorer work correctly with empty view groups
proto of auto-layout
fix for object_id in debug values
better error reporting for evaluation failures
implements drilling down into objects and arrays

requires a new version of Ensime
s-exp parser now correctly parses escapes in strings
minor patch to the watches GUI
got debug-string working

it was a bug in Ensime, which prevented it from operating correctly.
not bumping Ensime version, though, because it's already bumped a few
commits ago, so I'll just pack both fixes into a single pull request
watches now display this
watches can not optionally display classes of complex watchees
some refactoring in watch nodes
collection watches can now be lazily loaded (disable by default)
debug_to_string can fail not only by returning False, but also by returning None

this happens when it times out, as opposed to False, which comes from
an internal error in DebugManager in Ensime