This reverts commit 920f64c0.

Update required all CHECK_THROWS_AS macros to pass the exception type without reference, because this is now done by Catch2.

    If the array can be converted to basic_json::string_t, the overload
in this commit is not chosen.

Also moved all exception classes into the detail namespace and
introduced them via “using” into basic_json.

- tweaked a bit how `get<container<json>>` is handled
- added a from_json overload for forward list