To summarize:
 * str.encode does only str->bytes;
 * bytes.decode does only bytes-> str;
 * codecs.encode/decode do obj->obj;

The things that could go wrong are:
 1) the input type is wrong (i.e. the codec doesn't accept the type of the input);
 2) the input value is invalid;
 3) for str.encode/bytes.decode only, the output type is wrong (i.e. the codec returned a non-bytes/non-str object);

My patch only covers 3.  The four new exceptions suggested by Nick in msg187704 would cover the first 2 cases.
For str.encode/bytes.decode, if we knew the input type accepted by the codec we could also provide a better error message (e.g. "codecs accepts '...', not '...'; use ... instead"), but we don't.