Further improve promise exception

Author: sagikazarmark

src/Guzzle6Promise.php

@@ -58,17 +58,15 @@ public function __construct(PromiseInterface $promise, RequestInterface $request
             if ($reason instanceof HttplugException) {
                 $this->state = self::REJECTED;
                 $this->exception = $reason;
-
-                throw $this->exception;
-            }
-
-            if (!($reason instanceof GuzzleExceptions\GuzzleException)) {
-                throw new \RuntimeException('Invalid exception returned from Guzzle6', $reason);
+            } elseif ($reason instanceof GuzzleExceptions\GuzzleException) {
+                $this->state = self::REJECTED;
+                $this->exception = $this->handleException($reason, $request);
+            } elseif($reason instanceof \Exception) {
+                throw new \RuntimeException('Invalid exception returned from Guzzle6', 0, $reason);
+            } else {
+                throw new \UnexpectedValueException('Reason returned from Guzzle6 must be an Exception', 0, $reason);
             }
 
-            $this->state = self::REJECTED;
-            $this->exception = $this->handleException($reason, $request);
-
             throw $this->exception;
         });
     }