Error Handling: Startup

This is tracking adding some better error-handling in startup, to give the user feedback when there is an issue on startup. Coming from @Akin909 's comments in #1381:

A solution to this is that the changeDir function should always check that the directory it is passed is valid before attempting to switch to it. A more general issue is that I think every function or rather main startup process should have a try/catch or some form of error handling so that oni doesn't end up hanging without any feedback given back to the user

We should do the following:

• Wrap our 'start' call in a try/catch so that we can get errors
• Have some UI affordance for showing an error UI (perhaps in Shell?). This could show any errors logged, as well as give the option for opening up a debug window. We could also potentially show an error notification once #1358 is in.