Add new Err constructor to disambiguate the CantResolveAlts constructor.

(Ironic?) I did this to facilitate pretty printing of names when
Idris cannot disambiguate among alternatives.
Enable pretty printing of CantResolveAlts errors.

I undid the creation of the CantResolveTactics constructor, and instead
ensured all CantResolveAlts errors provide Names in some way.
Merge remote-tracking branch 'upstream/master' into cantResolveAlts

Conflicts:
	src/Idris/Output.hs
Merge pull request #1273 from bmsherman/cantResolveAlts

Change the type of the CantResolveAlts constructor.