Check if property exists before tweening

[KoBeWi]

When you try to tween a non-existent property, you get a confusing message about type mismatch between "nil" and target value type.
This PR adds a debug check whether a property exists, to print a clearer error.

[bitsawer]

I think adding a check with a better error message is good, but I'm a bit cautious about adding behavior that changes between debug and release builds. Could we do this check in both debug and release builds? It seems like a useful thing to do and it doesn't seem to be too heavy from performance perspective, either. And people often have to debug release build error logs too, so it also helps there.

[KoBeWi]

The behavior is the same (unless your final value is null for some reason...), only the error message is different. But indeed the check is cheap, so maybe it can be in release too, idk. The only time when it's relevant is when a) you don't test your project (this error is easy to catch) b) do some unsafe stuff with _get_property_list().

[timothyqiu]

The variable should not be named p_value as it's not a parameter :P

[KoBeWi]

It's property value xd

[akien-mga]

I'd suggest using prop_value to avoid confusion.

[YuriSizov]

I think it's fine like that, and we can always change it if someone signals that they need this check in production builds as well.

[akien-mga]

Thanks!