Handle TooManyRequests error (#53)

discogs.go

@@ -105,6 +105,8 @@ func request(path string, params url.Values, resp interface{}) error {
 		switch response.StatusCode {
 		case http.StatusUnauthorized:
 			return ErrUnauthorized
+		case http.StatusTooManyRequests:
+			return ErrTooManyRequests
 		default:
 			return fmt.Errorf("unknown error: %s", response.Status)
 		}

errors.go

@@ -16,10 +16,11 @@ func (e *Error) Error() string {
 
 // APIErrors
 var (
-	ErrUnauthorized         = &Error{"authentication required"}
 	ErrCurrencyNotSupported = &Error{"currency does not supported"}
-	ErrUserAgentInvalid     = &Error{"invalid user-agent"}
 	ErrInvalidReleaseID     = &Error{"invalid release id"}
 	ErrInvalidSortKey       = &Error{"invalid sort key"}
 	ErrInvalidUsername      = &Error{"invalid username"}
+	ErrTooManyRequests      = &Error{"too many requests"}
+	ErrUnauthorized         = &Error{"authentication required"}
+	ErrUserAgentInvalid     = &Error{"invalid user-agent"}
 )