Change the treatment of positions returned by PEP293

error handers in the Unicode codecs: Negative
positions are treated as being relative to the end of
the input and out of bounds positions result in an
IndexError.

Also update the PEP and include an explanation of
this in the documentation for codecs.register_error.

Fixes a small bug in iconv_codecs: if the position
from the callback is negative *add* it to the size
instead of substracting it.

From SF patch #677429.

pep-0293.txt

@@ -104,6 +104,10 @@ Specification
     specifies a new position within object, where (after encoding the
     replacement) the encoder will continue encoding.
 
+    Negative values for newpos are treated as being relative to
+    end of object. If newpos is out of bounds the encoder will raise
+    an IndexError.
+
     If the replacement string itself contains an unencodable character
     the encoder raises the exception object (but may set a different
     reason string before raising).