fix(google-maps): handle trying to access the map before it has been initialized

[crisbeto]

As things are set up at the moment, the Google Map object will be initialized once the API has loaded, however all of the methods on the GoogleMap component assume that the object will always be defined. This means that if any of the methods are called before it is initialized, we'll throw a null pointer error. These changes add a more readable error so people know what to do.

[jelbourn]

Rather than letting everything no-op when the map is undefined, it seems like throwing an error would be better (a real error, not just "Cannot read xxx of null") so people know what they're doing is invalid

[crisbeto]

Isn't that somewhat prone to race conditions? The consumer has no way of knowing when the maps has been initialized.

[jelbourn]

If we no-op, the user still hits the race condition, they just get no feedback that it happened and that their call did nothing. Without these methods being async and completing once the API loads, I think it has to error.

[crisbeto]

I've reworked it based on the discussion @jelbourn.

[jelbourn]

LGTM

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