[Q21.5] Handle UTF-8 more robustly; pass through information about incorrect sequences

[Aidan Kehoe]

 Ar an dara lá is fiche de mí Iúil, scríobh Stephen J. Turnbull: 

 > QUERY 21.5
 > 
 > Aidan Kehoe writes:
 > 
 >  > I’m running with this code,
 > 
 > Please document the in-buffer and on-disk formats properly, specifically
 > describing how to invert, or if they're not invertible, explain under
 > what conditions.

I’m not sure what you mean by that. With this patch, for every string X,
(encode-coding-string (decode-coding-string X 'utf-8) 'utf-8) is identical
to X, independent of whether X is valid UTF-8.

We should to notify the user about any file corruption on the UI level, and
prompt them about it when they save, for the sake of the spirit of the
Unicode implementation. 

-- 
On the quay of the little Black Sea port, where the rescued pair came once
more into contact with civilization, Dobrinton was bitten by a dog which was
assumed to be mad, though it may only have been indiscriminating. (Saki)