PR11207 groundwork: lexer::scan() produces tok_junk instead of throwing errors.

PR11207 groundwork: lexer::scan() produces tok_junk instead of throwing errors.

Handling parse_errors at scan time would get too complicated when there's
a preprocessing stage between the lexer and parser (and it's no longer
clear whether a given token will be used). Hence we produce specially marked
tok_junk which store a message to use when the token is encountered at
print_error() time.