Merge pull request #1909 from browner12/AB-fputs-failure-message

(3.0 branch) update exception message

phpseclib/Net/SSH2.php

@@ -4266,7 +4266,8 @@ class SSH2
 
         if (strlen($packet) != $sent) {
             $this->bitmap = 0;
-            throw new \RuntimeException("Only $sent of " . strlen($packet) . " bytes were sent");
+            $message = $sent === false ? ('Unable to write ' . strlen($packet) . ' bytes') : ("Only $sent of " . strlen($packet) . " bytes were sent");
+            throw new \RuntimeException($message);
         }
     }