Here's an example of one of these errors:
```
INVALID_KEY predicate py_cobjectnames(@py_cobject obj, string name)
The key set {obj} does not functionally determine all fields. Here is a
pair of tuples that agree on the key set but differ at index 1: Tuple 1
in row 63874: (72088,"u'<X>'") Tuple 2 in row 63875: (72088,"u'<?>'")
```
(Here, the substring `X` should really be the Unicode character U+FFFD,
but for some reason I'm not allowed to put that in this commit message.)
Inside the extractor, we assign IDs based on the string type (bytestring
or Unicode) and a hash of the UTF-8 encoded content of the string. In
this case, however, certain _different_ strings were receiving the same
hash, due to replacement characters in the encoding process.
In particular, we were converting unencodable characters to question
marks in one place, and to U+FFFD in another place. This caused a
discrepancy that lead to the dataset check error.
To fix this, we put in a custom error handler that always puts the
U+FFFD character in place of unencodable characters. With this, the
strings now agree, and hence there is no clash.
