WebDispatcher throws exceptions

[ValentinTrinque]

Due to the WebDispatcher behavior, Beluga core raises exceptions:

From the documentation:

In case the corresponding method doXXXX is not found on the api object, or if the URL is /, the action doDefault is used instead. An exception DispatchError.DENotFound("XXXX") is thrown if there is no default action (XXXX here being the placeholder for the URL part name).

In case the required part is missing, an exception DispatchError.DEMissing is thrown. In case the required part does not have the valid format, an exception DispatchError.DEInvalidValue is thrown.

The problem is we should allow the user to set his/her callback to manage this error by him/herself, especially for DEMissing because this is something which should be handled inside module and not the core by catching these exceptions.

[brissa-a]

Same topic as #182.