logutil: print error from WithError if found

Signed-off-by: Justin Chadwell <me@jedevc.com>

util/logutil/format.go

@@ -1,6 +1,7 @@
 package logutil
 
 import (
+	"bytes"
 	"fmt"
 	"strings"
 
@@ -12,5 +13,11 @@ type Formatter struct {
 }
 
 func (f *Formatter) Format(entry *logrus.Entry) ([]byte, error) {
-	return []byte(fmt.Sprintf("%s: %s\n", strings.ToUpper(entry.Level.String()), entry.Message)), nil
+	msg := bytes.NewBuffer(nil)
+	fmt.Fprintf(msg, "%s: %s", strings.ToUpper(entry.Level.String()), entry.Message)
+	if v, ok := entry.Data[logrus.ErrorKey]; ok {
+		fmt.Fprintf(msg, ": %v", v)
+	}
+	fmt.Fprintf(msg, "\n")
+	return msg.Bytes(), nil
 }