After reading through the linked thread, there are a few error message proposals:

Guido: "use data.encode('utf-8') if you want the data to be encoded in UTF-8". (Then of course the server might not like it.)

Andrew Barnert: A UnicodeEncodeError (or subclass of it?) with text like "HTTP body without encoding defaults to 'latin-1', which can't encode character '\u5555' in position 30: ordinal not in range(256)")

Paul Moore: Encode as ASCII and catch UnicodeEncodeError and re-raise as a TypeError "Unicode string supplied without an explicit encoding".

Emil, do you think any of these would help?