[skip changelog] Let sketch.Item.GetSourceStr return error

This adds an error return value to this method, which is currently
always nil. This prepares for actually returning errors later.

Author: matthijskooijman

Diff for: arduino/builder/sketch.go

@@ -221,18 +221,26 @@ func SketchMergeSources(sketch *sketch.Sketch) (int, string, error) {
 	mergedSource := ""
 
 	// add Arduino.h inclusion directive if missing
-	if !includesArduinoH.MatchString(sketch.MainFile.GetSourceStr()) {
+	mainSrc, err := sketch.MainFile.GetSourceStr()
+	if err != nil {
+		return 0, "", err
+	}
+	if !includesArduinoH.MatchString(mainSrc) {
 		mergedSource += "#include <Arduino.h>\n"
 		lineOffset++
 	}
 
 	mergedSource += "#line 1 " + QuoteCppString(sketch.MainFile.Path) + "\n"
-	mergedSource += sketch.MainFile.GetSourceStr() + "\n"
+	mergedSource += mainSrc + "\n"
 	lineOffset++
 
 	for _, item := range sketch.OtherSketchFiles {
+		src, err := item.GetSourceStr()
+		if err != nil {
+			return 0, "", err
+		}
 		mergedSource += "#line 1 " + QuoteCppString(item.Path) + "\n"
-		mergedSource += item.GetSourceStr() + "\n"
+		mergedSource += src + "\n"
 	}
 
 	return lineOffset, mergedSource, nil

Diff for: arduino/sketch/sketch.go

@@ -44,8 +44,8 @@ func NewItem(itemPath string) (*Item, error) {
 }
 
 // GetSourceStr returns the Source contents in string format
-func (i *Item) GetSourceStr() string {
-	return string(i.Source)
+func (i *Item) GetSourceStr() (string, error) {
+	return string(i.Source), nil
 }
 
 // ItemByPath implements sort.Interface for []Item based on