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
https://golang.org/cl/5294073

src/pkg/crypto/tls/alert.go

@@ -71,3 +71,7 @@ func (e alert) String() string {
 	}
 	return "alert(" + strconv.Itoa(int(e)) + ")"
 }
+
+func (e alert) Error() string {
+	return e.String()
+}