compress/lzw: be consistent with error message prefixes.

All the other error messages in this package are "lzw: etc".

Change-Id: Ie359a8912d213f92b15f02abc953127380503048
Reviewed-on: https://go-review.googlesource.com/11228Reviewed-by: David Symonds <dsymonds@golang.org>

src/compress/lzw/reader.go

@@ -213,7 +213,7 @@ func (d *decoder) flush() {
 	d.o = 0
 }
 
-var errClosed = errors.New("compress/lzw: reader/writer is closed")
+var errClosed = errors.New("lzw: reader/writer is closed")
 
 func (d *decoder) Close() error {
 	d.err = errClosed // in case any Reads come along