return errors instead of logrus.Fatal calls

This commit refactors the code to remove more or less
all occurences of logrus.Fatalf() from the code under
pkg/ except for app.go where all the errors are being
handled currently.

This is being done since random logrus.Fatalf() calls
all around the code was making handling the errors,
unit testing and troubleshooting a bit more painful.

logrus.Fatalf() calls are either replaced by
return errors.New("new error")
or
return errors.Wrap(err, "annonate error")
calls, and the function signatures are accordingly
changed to accomodate the new return values.

The unit tests which previously used to check
if logrus.Fatalf() calls worked fine have also
been fixed to only check for errors now.

Fixes #416

pkg/transformer/utils.go

@@ -30,6 +30,7 @@ import (
 
 	"path/filepath"
 
+	"github.com/pkg/errors"
 	"k8s.io/kubernetes/pkg/api"
 	"k8s.io/kubernetes/pkg/api/unversioned"
 	"k8s.io/kubernetes/pkg/runtime"
@@ -47,16 +48,16 @@ func RandStringBytes(n int) string {
 }
 
 // CreateOutFile creates the file to write to if --out is specified
-func CreateOutFile(out string) *os.File {
+func CreateOutFile(out string) (*os.File, error) {
 	var f *os.File
 	var err error
 	if len(out) != 0 {
 		f, err = os.Create(out)
 		if err != nil {
-			log.Fatalf("error creating file: %v", err)
+			return nil, errors.Wrap(err, "error creating file, os.Create failed")
 		}
 	}
-	return f
+	return f, nil
 }
 
 // ParseVolume parses a given volume, which might be [name:][host:]container[:access_mode]
@@ -151,7 +152,7 @@ func TransformData(obj runtime.Object, GenerateJSON bool) ([]byte, error) {
 }
 
 // Print either prints to stdout or to file/s
-func Print(name, path string, trailing string, data []byte, toStdout, generateJSON bool, f *os.File) string {
+func Print(name, path string, trailing string, data []byte, toStdout, generateJSON bool, f *os.File) (string, error) {
 	file := ""
 	if generateJSON {
 		file = fmt.Sprintf("%s-%s.json", name, trailing)
@@ -160,20 +161,20 @@ func Print(name, path string, trailing string, data []byte, toStdout, generateJS
 	}
 	if toStdout {
 		fmt.Fprintf(os.Stdout, "%s\n", string(data))
-		return ""
+		return "", nil
 	} else if f != nil {
 		// Write all content to a single file f
 		if _, err := f.WriteString(fmt.Sprintf("%s\n", string(data))); err != nil {
-			log.Fatalf("Failed to write %s to file: %v", trailing, err)
+			return "", errors.Wrap(err, "f.WriteString failed, Failed to write %s to file: "+trailing)
 		}
 		f.Sync()
 	} else {
 		// Write content separately to each file
 		file = filepath.Join(path, file)
 		if err := ioutil.WriteFile(file, []byte(data), 0644); err != nil {
-			log.Fatalf("Failed to write %s: %v", trailing, err)
+			return "", errors.Wrap(err, "Failed to write %s: "+trailing)
 		}
 		log.Printf("file %q created", file)
 	}
-	return file
+	return file, nil
 }