aal: respond to when libMediaPlayer fails to create a session
There used to be a try-catch here which is removed when this is re-written to use the Qt version of the library, presumbly because the new code would not throw. Well, with [1] now it will, and we better prepare for that.
And since there's a few places which doesn't handle player session being absent well, also hardening those places.
Related: media-hub#63 (closed)