Add error handling

Added error handling around ineffectual err assignments. Please review
thoroughly.

viper.go

@@ -1469,6 +1469,9 @@ func (v *Viper) marshalWriter(f afero.File, configType string) error {
 
 	case "hcl":
 		b, err := json.Marshal(c)
+		if err != nil {
+			return ConfigMarshalError{err}
+		}
 		ast, err := hcl.Parse(string(b))
 		if err != nil {
 			return ConfigMarshalError{err}

viper_test.go

@@ -952,6 +952,7 @@ func TestDirsSearch(t *testing.T) {
 	v.SetDefault(`key`, `default`)
 
 	entries, err := ioutil.ReadDir(root)
+	assert.Nil(t, err)
 	for _, e := range entries {
 		if e.IsDir() {
 			v.AddConfigPath(e.Name())