Coverage checker fix

If a candidate case fails with a 'recoverable' unification error (i.e.
one which could be fixed by allowing more reduction somewhere) then it
should be considered valid for coverage checking purposes.

Fixes #1069 (and possibly some others, not checked yet!)

(There is, I think a better fix, involving better generation of
candidate cases, which I will leave for another time)
Merge branch 'master' of github.com:idris-lang/Idris-dev