engine: Detect bad return value from app start() method
engine: Detect bad return value from app start() method

Raise a specific error instead of allowing a bad value to propagate in
the engine (leading to a more confusing error message).

plajjan/snabbswitch,heryii/snabb,wingo/snabbswitch,kbara/snabb,Igalia/snabbswitch,dpino/snabb,snabbco/snabb,eugeneia/snabb,eugeneia/snabbswitch,wingo/snabb,Igalia/snabb,aperezdc/snabbswitch,kbara/snabb,SnabbCo/snabbswitch,dpino/snabbswitch,heryii/snabb,lukego/snabbswitch,Igalia/snabbswitch,heryii/snabb,wingo/snabbswitch,kbara/snabb,kbara/snabb,justincormack/snabbswitch,SnabbCo/snabbswitch,snabbco/snabb,mixflowtech/logsensor,eugeneia/snabb,mixflowtech/logsensor,kbara/snabb,eugeneia/snabbswitch,SnabbCo/snabbswitch,wingo/snabb,dpino/snabbswitch,eugeneia/snabb,plajjan/snabbswitch,wingo/snabbswitch,alexandergall/snabbswitch,aperezdc/snabbswitch,wingo/snabb,Igalia/snabb,plajjan/snabbswitch,alexandergall/snabbswitch,mixflowtech/logsensor,eugeneia/snabb,alexandergall/snabbswitch,aperezdc/snabbswitch,dpino/snabb,lukego/snabbswitch,lukego/snabbswitch,alexandergall/snabbswitch,eugeneia/snabb,snabbco/snabb,dpino/snabb,eugeneia/snabbswitch,dpino/snabb,andywingo/snabbswitch,Igalia/snabbswitch,eugeneia/snabb,Igalia/snabbswitch,mixflowtech/logsensor,wingo/snabb,snabbco/snabb,wingo/snabb,Igalia/snabb,Igalia/snabb,Igalia/snabb,lukego/snabbswitch,lukego/snabb,heryii/snabb,andywingo/snabbswitch,mixflowtech/logsensor,eugeneia/snabb,justincormack/snabbswitch,alexandergall/snabbswitch,snabbco/snabb,Igalia/snabb,andywingo/snabbswitch,lukego/snabb,andywingo/snabbswitch,dpino/snabb,Igalia/snabb,justincormack/snabbswitch,Igalia/snabb,heryii/snabb,kbara/snabb,SnabbCo/snabbswitch,dpino/snabbswitch,wingo/snabbswitch,snabbco/snabb,alexandergall/snabbswitch,alexandergall/snabbswitch,dpino/snabb,lukego/snabb,aperezdc/snabbswitch,eugeneia/snabbswitch,dpino/snabbswitch,snabbco/snabb,eugeneia/snabb,snabbco/snabb,Igalia/snabbswitch,dpino/snabb,plajjan/snabbswitch,heryii/snabb,wingo/snabb,alexandergall/snabbswitch,lukego/snabb,justincormack/snabbswitch