Pass the transfer exception through as "previous" (#13)

- Provides access to server response to help identify errors

src/Client.php

@@ -32,7 +32,7 @@ public function query(string $query, array $variables = []): Response
         try {
             $response = $this->httpClient->request('POST', '', $options);
         } catch (TransferException $e) {
-            throw new \RuntimeException('Network Error.');
+            throw new \RuntimeException('Network Error.'.$e->getMessage(), 0, $e);
         }
 
         return $this->responseBuilder->build($response);

tests/ClientTest.php

@@ -31,6 +31,28 @@ public function testSimpleQueryWhenHasNetworkErrors()
         $this->client->query($query);
     }
 
+    public function testCanRetrievePreviousExceptionWhenSimpleQueryHasErrors()
+    {
+        $previousException = null;
+        try {
+            $originalException = new \GuzzleHttp\Exception\ServerException(
+                'Server side error',
+                $this->createMock(\Psr\Http\Message\RequestInterface::class)
+            );
+
+            $this->httpClient->expects($this->once())
+                ->method('request')
+                ->willThrowException($originalException);
+
+            $query = $this->getSimpleQuery();
+            $this->client->query($query);
+        } catch (\Exception $e) {
+            $previousException = $e->getPrevious();
+        } finally {
+            $this->assertSame($originalException, $previousException);
+        }
+    }
+
     public function testSimpleQueryWhenInvalidJsonIsReceived()
     {
         $query = $this->getSimpleQuery();