The Writer logic was not consistent about when an IO error would
persist across multiple calls on Writer's methods.

Thus, to make the error handling more consistent we always check
the persistent state of the error prior to every exported method
call, and return an error if set. Otherwise, it is the responsibility
of every exported method to persist any fatal errors that may occur.

As a simplification, we can remove the close field since that
information can be represented by simply storing ErrWriteAfterClose
in the err field.

Change-Id: I8746ca36b3739803e0373253450db69b3bd12f38
Reviewed-on: https://go-review.googlesource.com/55590
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>