Fix panic when submit not given args

Previously, submit panics when not given args due when trying to index
an empty array:

    exercise, err := ctx.exercise(submitPaths[0])

cmd/submit.go

@@ -290,6 +290,9 @@ type submitValidator struct {
 
 // filesExistAndNotADir checks that each file exists and is not a directory.
 func (s submitValidator) filesExistAndNotADir(submitPaths []string) error {
+	if len(submitPaths) == 0 {
+		return fmt.Errorf("usage: %s submit FILE1 [FILE2 ...]", BinaryName)
+	}
 	for _, path := range submitPaths {
 		path, err := filepath.Abs(path)
 		if err != nil {

cmd/submit_test.go

@@ -47,6 +47,30 @@ func TestSubmitWithoutWorkspace(t *testing.T) {
 	}
 }
 
+func TestSubmitWithoutArgs(t *testing.T) {
+	tmpDir, err := ioutil.TempDir("", "submit-no-args")
+	defer os.RemoveAll(tmpDir)
+	assert.NoError(t, err)
+
+	v := viper.New()
+	v.Set("token", "abc123")
+	v.Set("workspace", tmpDir)
+	v.Set("apibaseurl", "http://api.example.com")
+
+	cfg := config.Config{
+		Persister:       config.InMemoryPersister{},
+		UserViperConfig: v,
+		DefaultBaseURL:  "http://example.com",
+	}
+
+	var noCLIArguments []string
+
+	err = runSubmit(cfg, pflag.NewFlagSet("fake", pflag.PanicOnError), noCLIArguments)
+	if assert.Error(t, err) {
+		assert.Contains(t, err.Error(), "usage")
+	}
+}
+
 func TestSubmitNonExistentFile(t *testing.T) {
 	tmpDir, err := ioutil.TempDir("", "submit-no-such-file")
 	defer os.RemoveAll(tmpDir)