hugolib: Fix reloading corner cases for shortcodes

This commit fixes two different, but related issues:

1) Live-reload when a new shortcode was defined in the content file before the shortcode itself was created.
2) Live-reload when a newly defined shortcode changed its "inner content" status.

This commit also improves the shortcode related error messages to include the full path to the content file in question.

Fixes #3156

hugolib/shortcode_test.go

@@ -352,7 +352,8 @@ func TestExtractShortcodes(t *testing.T) {
 			return nil
 		})
 
-		content, shortCodes, err := extractShortcodes(this.input, p)
+		s := newShortcodeHandler()
+		content, err := s.extractShortcodes(this.input, p)
 
 		if b, ok := this.expect.(bool); ok && !b {
 			if err == nil {
@@ -371,6 +372,8 @@ func TestExtractShortcodes(t *testing.T) {
 			}
 		}
 
+		shortCodes := s.shortcodes
+
 		var expected string
 		av := reflect.ValueOf(this.expect)
 		switch av.Kind() {