Added more error handling and necessary functions

plainText.go

@@ -1,6 +1,9 @@
 package atom
 
-import "fmt"
+import (
+	"errors"
+	"fmt"
+)
 
 type PlainText struct {
 	*CommonAttributes
@@ -11,6 +14,15 @@ type PlainText struct {
 // isText checks whether the PlainText is a Text. It returns a bool.
 func (p *PlainText) isText() bool { return true }
 
+// newPlainText creates a new PlainText. It returns a *PlainText and an error.
+func newPlainText(textType, content string) (*PlainText, error) {
+	if content == "" {
+		return nil, errors.New("error creating new plain text: content string empty")
+	}
+
+	return &PlainText{Type: textType, Text: content}, nil
+}
+
 // Check checks the PlainText for incompatibilities with RFC4287. It returns an
 // error.
 func (p *PlainText) Check() error {