The main motivation for this is unit tests, which contain a large macro
for pretty-printing std::error_code, and this macro is duplicated in
every file that needs to do this. However, the functionality may be
useful elsewhere too.
In this patch I have reimplemented the existing ASSERT_NO_ERROR macros
to reuse the new functionality, but I have kept the macro (as a
one-liner) as it is slightly more readable than ASSERT_EQ(...,