Get rid of error_type field in Token.

It is unnecessary for all tokens to store this. Instead just make a few error kinds and have a lookup table for their meaning.

Start by locating the Token definition and every use of its error_type field in the tokenizer. Determine the required error kinds and the lookup-table meanings before changing the representation. Done means tokens no longer store error_type and error meanings remain available through the new lookup design, with existing behavior preserved.

Written by the indexing model from the issue text.