(Except they're not wired up in xunit, so they do nothing. Everything still goes through IErrorMessage)

Class and collection cleanup failure now uses xunit2 error messages.
Collection cleanup failure fails all associated classes and methods.
IErrorMessage is now xunit1 only and includes enough context to report
errors without requiring shared state (although it assumes it has to
fail a class). Requires build 2716 + some smaller fixes (xunit/xunit#143
and xunit/xunit#144)