The attached proof of concept catches Type/ValueError in str.encode and raises another exception with a better message:
>>> 'example'.encode('hex_codec')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: invalid input type for hex_codec codec ('str' does not support the buffer interface)

(note: the patch doesn't handle the exception chaining yet and probably leaks.)

If Nick proposal in msg187704 is accepted, this should become a codecs.EncodeTypeError.  The same should then be done for bytes.decode and for codecs.encode/decode.