src/utilities.lisp, src/class-finalize-impl.lisp: Improve error reports.

src/utilities.lisp, src/class-finalize-impl.lisp: Improve error reports.

  * Arrange for `inconsistent-merge-error' to present the list of
    candidates more pleasantly for human reading.

  * Give `merge-lists' a new argument to turn items into presentable
    names.

  * Make a wrapper function for merging lists of classes to apply the
    right kind of presentation.