json_pack: Improve handling of formats with '?' and '*'.

When NULL is received for an optional argument we should not set an
error message as this would block later error messages. If NULL is
received for a non-optional string we should set has_error. Set
has_error for UTF-8 errors to ensure optional strings with UTF-8
errors are not replaced with json_null(). Use 'purpose' argument in
NULL error messages of read_string.

Add error handling and tests for invalid formats where '+', '#', or '%'
is used on an optional string 's?' or 's*'.

Fix NULL string error messages to use 'purpose'.

Refactor skipping of '*' token, this is now handled by read_string and
pack_object_inter. This allows invalid format strings such as 's*#' and
's*+' to produce error messages.

Fixes #437