all: rename os.Error to error in various non-code contexts

R=adg
CC=golang-dev
https://golang.org/cl/5328062

conn.go

@@ -828,7 +828,7 @@ func (c *Conn) OCSPResponse() []byte {
 }
 
 // VerifyHostname checks that the peer certificate chain is valid for
-// connecting to host.  If so, it returns nil; if not, it returns an os.Error
+// connecting to host.  If so, it returns nil; if not, it returns an error
 // describing the problem.
 func (c *Conn) VerifyHostname(host string) error {
 	c.handshakeMutex.Lock()