fix(notifications): handle Error type messages

[andrewazores]

Fixes #304

The root cause of the bug is that services and components may attempt to display a notification with an Error object as the message to display to the user. This fails because the rendered child component (content of the graphical notification) must be a string. This is a bit tricky because the Notification data type specifies that the message property should be a string, however many/most places where an Error object may be obtained it is typed as 'any', which breaks some of the TS compiler's type-checking guarantees at compile time. To work around this, I apply a simple fix to the notifications service: do a runtime check of the message type, and if it isn't an actual string, apply "JSON.stringify" to serialize it to a string before it is displayed.

[ebaron]

No more white screen after this fix! There's 3 error notifications, which might not mean anything to users, but it works.

[andrewazores]

I think that's to be expected at this point. The first one looks like it comes from the frontend's HTTP service, which automatically displays notifications on 500s. The second is from the component that made that request, trying to give more detail about the specific failure. And the last one would have been there normally due to the messaging service connection failure due to also lacking auths.