sdk should provide some common way to check specific error

[wanliqun]

As @boqiu metioned in https://github.com/Conflux-Dev/conflux-infura/pull/69#discussion_r655829246, sdk should provide some common way for client application to check if the return error is some specific error.

        epochReceipts, err := cfx.GetEpochReceiptsByPivotBlockHash(pivotHash)
	if err != nil {
		errStr := strings.ToLower(err.Error())

		// pivot hash assumption failed, must be pivot switched
		if strings.Contains(errStr, "pivot assumption failed") || strings.Contains(errStr, "block not found") {
                      // handle pivot switch
                 }
         }

This issue is urgent, and is blocking infura && scan, please take a look and discuss about it.

@Pana @wangdayong228