[MERGE] allow recovering bad texts from knits

[Andrew Bennetts]

Robert Collins wrote:
> When a line delta has a bad basis - which baz imports can create- it can
> be useful to be able to inspect the result of the text delta application
> even when the sha1 is wrong and it fails validation.
> 
> This patch adds a attribute and specific exception allowing this to be
> done.

This seems ok to me.  My only comment is that the exception message has less
pretty formatting now (no newlines and indents), but I guess this is
intentional?  (i.e. you prefer it this way?)

bb:approve

-Andrew.