Sending a large error should mention that we're out of space

The returned error now includes the underlying error from the typed
writer, resulting in:
> failed to create outbound error frame: no more room in buffer

connection.go

@@ -494,7 +494,7 @@ func (c *Connection) SendSystemError(id uint32, span Span, err error) error {
 			LogField{"id", id},
 			ErrField(err),
 		).Warn("Couldn't create outbound frame.")
-		return fmt.Errorf("failed to create outbound error frame")
+		return fmt.Errorf("failed to create outbound error frame: %v", err)
 	}
 
 	// When sending errors, we hold the state rlock to ensure that sendCh is not closed

connection_test.go

@@ -37,6 +37,7 @@ import (
 	"github.com/uber/tchannel-go/testutils"
 	"github.com/uber/tchannel-go/testutils/testreader"
 	"github.com/uber/tchannel-go/tos"
+	"github.com/uber/tchannel-go/typed"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -626,6 +627,24 @@ func TestWriteArg3AfterTimeout(t *testing.T) {
 	})
 }
 
+func TestLargeSendSystemError(t *testing.T) {
+	testutils.WithTestServer(t, nil, func(ts *testutils.TestServer) {
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+
+		opts := testutils.NewOpts().AddLogFilter("Couldn't create outbound frame.", 1)
+		client := ts.NewClient(opts)
+		conn, err := client.Connect(ctx, ts.HostPort())
+		require.NoError(t, err, "Connect failed")
+
+		largeErr := errors.New(strings.Repeat("1234567890", 10000))
+		err = conn.SendSystemError(1, Span{}, largeErr)
+		require.Error(t, err, "Expect err")
+		assert.Contains(t, err.Error(), typed.ErrBufferFull.Error())
+		assert.Contains(t, err.Error(), "wat")
+	})
+}
+
 func TestWriteErrorAfterTimeout(t *testing.T) {
 	// TODO: Make this test block at different points (e.g. before, during read/write).
 	testutils.WithTestServer(t, nil, func(ts *testutils.TestServer) {