Type error: expected Exception, got string with mock + log middleware #6

The MockMiddleware creates a RejectedPromise with a string reason in https://github.com/csarrazi/guzzle-cache-middleware/blob/master/src/MockMiddleware.php#L47 if the cache is not found. The log middleware then fails because the MessageFormatter requires an exception in https://github.com/guzzle/guzzle/blob/master/src/MessageFormatter.php#L70

It seems like a rejection reason does not necessarily need to be an exception because the reason is never typehinted in guzzle/promises. In this case the bug is more in Guzzle itself that the log middleware cannot deal with that.