The 'deriving' mechanism that infers the context for
an instance declarations was going into a loop, as a
result of an instance like
       instance C a b => D [a]
where the 'b' isn't mentioned in the head.

This patch identifies those cases.  I also needed to make
TcErrors generate a suitable error message.  On the way
I improved the reporting of "ambiguous" variables;
for example arrowfail001 now generates a better message.