Improve error message on un-satisfied import
Consider import M( C( a,b,c ) ) where class C is defined as module M where class C x where a :: blah c :: blah Tnen (Trac #15413) we'd like to get an error message only about failing to import C( b ), not C( a,b,c ). This was fairly easy (and local) to do. Turned out that the existing tests mod81 and mod91 are adequate tests for the feature.
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