liblokatt: filter: better error messages

[amhk]

Have create_filter accept an optional out-paramter (of type struct strbuf *). If there is an error while creating the filter, the strbuf should contain a message intended to be shown to the end user.

The lexer already contains logic to pinpoint what character in the input caused problems. The parser needs to be refined -- perhaps add more logic to tokens about where in the input they came from?