crypto/cipher: don't persist errors in StreamWriter.

I messed this up from the beginning. The receiver isn't a pointer so
setting Err is useless. In order to maintain the API, just remove the
superfluous code.

Fixes issue 4657.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7161043
crypto/x509: return a better error when we fail to load system roots.

R=golang-dev, krautz, rsc
CC=golang-dev
https://codereview.appspot.com/7157044