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

Related to #34

errors/error.go

@@ -9,6 +9,7 @@ import (
 )
 
 // CustomError implements the following interfaces:
+//
 //	error
 //	github.com/pkg/errors: Cause
 type CustomError struct {
@@ -251,7 +252,7 @@ func IsRetryableError(err error) bool {
 	if code == 0 {
 		return false
 	}
-	
+
 	// 429 should always retry
 	if code == http.StatusTooManyRequests {
 		return true
@@ -262,5 +263,10 @@ func IsRetryableError(err error) bool {
 		return true
 	}
 
+	// Explicitly check for and return false for any 400s as well
+	if code >= 400 && code < 500 {
+		return false
+	}
+
 	return false
 }