Improve error handling (#7)

* Improve documentation - add navigation link

* Improve error handling

* Version bump to 1.0.1

* Optimize the code for the bug fix

* Improve code

composer.json

@@ -3,7 +3,7 @@
     "description": "SumUp eCom SDK for PHP",
     "type": "library",
     "license": "proprietary",
-    "version": "1.0.0",
+    "version": "1.0.1",
     "keywords": ["sumup", "sdk", "payment processing", "ecommerce", "payment", "checkout"],
     "homepage": "https://developer.sumup.com",
     "authors": [

src/SumUp/HttpClients/Response.php

@@ -95,12 +95,36 @@ protected function parseResponseForErrors()
             throw new SumUpValidationException($invalidFields, $this->httpResponseCode);
         }
         if ($this->httpResponseCode >= 500) {
-            $message = (!is_null($this->body) && !is_null($this->body->message)) ? $this->body->message : $this->body;
+            $message = $this->parseErrorMessage('Server error');
             throw new SumUpServerException($message, $this->httpResponseCode);
         }
         if ($this->httpResponseCode >= 400) {
-            $message = (!is_null($this->body) && !is_null($this->body->message)) ? $this->body->message : $this->body;
+            $message = $this->parseErrorMessage('Client error');
             throw new SumUpResponseException($message, $this->httpResponseCode);
         }
     }
+
+    /**
+     * Return error message.
+     * 
+     * @param string $defaultMessage
+     * 
+     * @return string
+     */
+    protected function parseErrorMessage($defaultMessage = '')
+    {
+        if (is_null($this->body)) {
+            return $defaultMessage;
+        }
+
+        if (isset($this->body->message)) {
+            return $this->body->message;
+        }
+
+        if (isset($this->body->error_message)) {
+            return $this->body->error_message;
+        }
+
+        return $defaultMessage;
+    }
 }