fix error handling in the TLS crypto setup

There are two ways that an error can occur during the handshake:
1. as a return value from qtls.Handshake()
2. when new data is passed to the crypto setup via HandleData()
We need to make sure that the RunHandshake() as well as HandleData()
both return if an error occurs at any step during the handshake.

crypto_stream_manager_test.go

@@ -1,8 +1,6 @@
 package quic
 
 import (
-	"errors"
-
 	"github.com/golang/mock/gomock"
 	"github.com/lucas-clemente/quic-go/internal/protocol"
 	"github.com/lucas-clemente/quic-go/internal/wire"
@@ -54,12 +52,4 @@ var _ = Describe("Crypto Stream Manager", func() {
 		cs.EXPECT().HandleData([]byte("foobar"), protocol.EncryptionHandshake)
 		Expect(csm.HandleCryptoFrame(f, protocol.EncryptionHandshake)).To(Succeed())
 	})
-
-	It("returns the error if handling crypto data fails", func() {
-		testErr := errors.New("test error")
-		f := &wire.CryptoFrame{Data: []byte("foobar")}
-		cs.EXPECT().HandleData([]byte("foobar"), protocol.EncryptionHandshake).Return(testErr)
-		err := csm.HandleCryptoFrame(f, protocol.EncryptionHandshake)
-		Expect(err).To(MatchError(testErr))
-	})
 })