Scanner.Next should return an actual error

[db47h]

The idea is to distinguish lexing errors from I/O errors.

On the other hand, if an I/O error occurs, s.abort is set to true, so any subsequent calls to next will return an EOF token.

Clients that need to distinguish error types may consider the built-in Error token as fatal and implement their own soft error token in their ragel file.

Something possibly more versatile is to have any returned error token added to a list of errors (with the actual error interface) and add ErrorCount and Errors method to Scanner.

[db47h]

After much consideration and extensive use with a couple of parsers, it turns out that an error from the lexer, whether due to incorrect input or I/O, will result in a parsing failure. Adding different error cases only complicates the parser implementation for no benefit.