Resolve "Cater for errors being nil and update checks for retryable errors"

Related to #34

errors/error.go

@@ -263,10 +263,36 @@ func IsRetryableError(err error) bool {
 		return true
 	}
 
-	// Explicitly check for and return false for any 400s as well
+	return false
+}
+
+func IsRetryableErrorOrShouldFail(err error) (shouldRetry bool, shouldFail bool) {
+	if err == nil {
+		return false, false
+	}
+
+	// Check for retryable
+	if IsRetryableError(err) {
+		return true, false
+	}
+
+	code := HTTPCode(err)
+
+	// If no HTTP code in the error, just fail but don't retry
+	if code == 0 {
+		return false, true
+	}
+
+	// Explicitly check for 400s, and don't retry or fail
 	if code >= 400 && code < 500 {
-		return false
+		return false, false
 	}
 
-	return false
+	// Explicitly check for 200s, for a fail-safe
+	if code >= 200 && code < 300 {
+		return false, false
+	}
+
+	// If we got here, we should fail normally
+	return false, true
 }