add github.com/pkg/errors to glide and vendor

This adds github.com/pkg/errors to glide.yaml followed
by glide and glide-vc commands. The github.com/pkg/errors
package is currently required mainly for the errors.Wrap()
and errors.New() methods, since this lets us to annotate the
errors while passing the error message up the call stack.

vendor/github.com/Sirupsen/logrus/formatter.go

@@ -31,15 +31,18 @@ type Formatter interface {
 // It's not exported because it's still using Data in an opinionated way. It's to
 // avoid code duplication between the two default formatters.
 func prefixFieldClashes(data Fields) {
-	if t, ok := data["time"]; ok {
-		data["fields.time"] = t
+	_, ok := data["time"]
+	if ok {
+		data["fields.time"] = data["time"]
 	}
 
-	if m, ok := data["msg"]; ok {
-		data["fields.msg"] = m
+	_, ok = data["msg"]
+	if ok {
+		data["fields.msg"] = data["msg"]
 	}
 
-	if l, ok := data["level"]; ok {
-		data["fields.level"] = l
+	_, ok = data["level"]
+	if ok {
+		data["fields.level"] = data["level"]
 	}
 }