Better exception message when class not found

[Nyholm]

When we do not find a factory we throw an exception like this:

"No factories found. Install php-http/message to use Guzzle or Diactoros factories."

This is not correct. You could have php-http/message installed and still get this error. We should explain better that you need php-http/message and an implementation of PSR-7.

Same with the HttpAsyncClientDiscovery and HttpClientDiscovery, atm we get an exception saying "Not found". This could also be improved.

[sagikazarmark]

You are right, the message is incorrect.

As for the other discoveries: We could extend the message to say that the given resource type has no available implementation.

[Nyholm]

👍