add CamelExceptions to several functions. Use camel_session_new to

add CamelExceptions to several functions. Use camel_session_new to

        * camel-session.h:
        * camel-session.c: add CamelExceptions to several functions. Use
        camel_session_new to initialize the session and URL fields of
        created CamelStores as appropriate.

        * camel-store.h:
        * camel-store.c
        * camel-service.h:
        * camel-service.c: Move the session and url (and associated
        functions) from CamelStore to CamelService. Add url_flags to
        CamelService so subclasses can specify which URL components
        are mandatory for them. Add camel_session_new for
        camel_session_get_store* to use.

        * providers/mbox/camel-mbox-folder.c:
        * providers/mbox/camel-mbox-store.c:
        * providers/mbox/camel-mbox-store.h: Update for above changes.