all: const instead of var for errors

attributes.go

@@ -405,7 +405,7 @@ func (m *Message) GetErrorCode() (int, []byte, error) {
 	return code, reason, nil
 }
 
-var (
+const (
 	// ErrAttributeNotFound means that there is no such attribute.
 	ErrAttributeNotFound Error = "Attribute not found"
 

stun.go

@@ -321,7 +321,7 @@ func IsMessage(b []byte) bool {
 		binary.BigEndian.Uint32(b[4:8]) == magicCookie
 }
 
-var (
+const (
 	// ErrUnexpectedHeaderEOF means that there were not enough bytes in
 	// m.Raw to read header.
 	ErrUnexpectedHeaderEOF Error = "unexpected EOF: not enough bytes to read header"