1.0.28.39: more error reporting tweakery

1.0.28.39: more error reporting tweakery

 * Now that *CURRENT-PATH* is bound first by PROCESS-TOPLEVEL-FORM
   make FIND-ERROR-CONTEXT prefer NODE-SOURCE-PATH *CURRENT-PATH* --
   otherwise the more accurate source path in the node is never used.

   (Experimentally it seems that *CURRENT-PATH* is never more
   accurate.)

   Everything seems OK, but the whole source path mechanism could
   probably us an audit, and possibly some refactoring.

 * ADD-TEST-CONSTRAINTS should bind *COMPILER-ERROR-CONTEXT* around
   the call to SPECIALIZER-TYPE in case of undefined types. (Left out
   from last patch by accident.)