crypto/tls: add Error method to alert
crypto/tls: add Error method to alert

alerts get used as both values and errors.
Rather than introduce an alertError wrapper,
this CL just adds an Error method, which will
satisfy the error interface when the time comes.

R=agl, bradfitz
CC=golang-dev
http://codereview.appspot.com/5294073
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go