rename add_parse_error() to avoid name conflicts and support amalgamation file like sqlite

The amalgamation file looks like below (so that the user can just add one file to the embedder project instead of linking a library):

#include "attribute.c"
#include "char_ref.c"
#include "error.c"
#include "parser.c"
#include "string_buffer.c"
#include "string_piece.c"
#include "tag.c"
#include "tokenizer.c"
#include "utf8.c"
#include "util.c"
#include "vector.c"