There are three classes of methods/functions called Error:

a) The Error method in the just introduced error interface
b) Error methods that create or report errors (http.Error, etc)
c) Error methods that return errors previously associated with
   the receiver (Tokenizer.Error, rows.Error, etc).

This CL introduces the convention that methods in case (c)
should be named Err.

The reasoning for the change is:

- The change differentiates the two kinds of APIs based on
  names rather than just on signature, unloading Error a bit
- Err is closer to the err variable name that is so commonly
  used with the intent of verifying an error
- Err is shorter and thus more convenient to be used often
  on error verifications, such as in iterators following the
  convention of the sql package.

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5327064