Improve error message when dmpstruct.h needs to be updated

Improve error message when dmpstruct.h needs to be updated

* src/pdumper.c (dump_cons, dump_interval_tree, dump_string)
(dump_marker, dump_overlay, dump_finalizer)
(dump_bignum, dump_float, dump_fwd_int, dump_fwd_bool)
(dump_fwd_obj, dump_fwd_buffer_obj, dump_fwd_kboard_obj)
(dump_fwd, dump_blv, dump_symbol, dump_vectorlike_generic)
(dump_hash_table, dump_buffer, dump_bool_vector, dump_subr)
(dump_vectorlike, dump_object, dump_charset) [CHECK_STRUCTS]:
Make the error message more specific where to find the comment
to CHECK_STRUCTS.