improve exception handling in TorVPN Service

catch onionmasq exceptions directly on the coroutine scope, use launch instead of async therefore to start coroutines

Some context: Before the onionmasq exceptions were not properly handled, since they got stored in async's Deferred object, which we silently dropped here (we didn't assign a variable to the async block and awaited for the result in order to handle either the result or the exception).

launch is more of a fire-and-forget approach, any exceptions need to be handled directly in the code block, otherwise the app would crash.