crypto/tls: check errors from (*Conn).writeRecord

This promotes a connection hang during TLS handshake to a proper error.
This doesn't fully address #14539 because the error reported in that
case is a write-on-socket-not-connected error, which implies that an
earlier error during connection setup is not being checked, but it is
an improvement over the current behaviour.

Updates #14539.

Change-Id: I0571a752d32d5303db48149ab448226868b19495
Reviewed-on: https://go-review.googlesource.com/19990
Reviewed-by: Adam Langley <agl@golang.org>

handshake_server_test.go

@@ -80,7 +80,10 @@ func testClientHelloFailure(t *testing.T, serverConfig *Config, m handshakeMessa
 		cli.writeRecord(recordTypeHandshake, m.marshal())
 		c.Close()
 	}()
-	err := Server(s, serverConfig).Handshake()
+	hs := serverHandshakeState{
+		c: Server(s, serverConfig),
+	}
+	_, err := hs.readClientHello()
 	s.Close()
 	if len(expectedSubStr) == 0 {
 		if err != nil && err != io.EOF {