move all error things to new qerr package, replacing errorcodes

qerr/quic_error_test.go

@@ -0,0 +1,15 @@
+package qerr_test
+
+import (
+	"github.com/lucas-clemente/quic-go/qerr"
+
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("Quic error", func() {
+	It("has a string representation", func() {
+		err := qerr.Error(qerr.InternalError, "foobar")
+		Expect(err.Error()).To(Equal("InternalError: foobar"))
+	})
+})