This allows the use of ~ts/~tp/~tw in the formatting fun to
sys:handle_debug/4.

This reverts commit eaf8ca41dfa4850437ad270d3897399c9358ced0.

Return error tuple on unicode normalization functions

Prior to this patch, the normalization functions in the
unicode module would raise a function clause error for
non-utf8 binaries.

This patch changes it so it returns {error, SoFar, Invalid}
as characters_to_binary and characters_to_list does in
the unicode module.

Note string:next_codepoint/1 and string:next_grapheme had
to be changed accordingly and also return an error tuple.

* raimo/gen_statem-dev:
  Fix documentation details