[Soap/Server] add debug mode

When an exception is raised the \Zend\Soap\Server catch it and check if this is an authorized exception. 
If not, and for security reason, it send an "Unknow error" message. 
And if in production its a sane behaviour, its really annoying during development & tests process.

So I propose to add a setDebugMode method, when activated, send all exceptions to the client.

The code is unit-tested.