Allow accessing and changing exception value in `before_send` handler

[limbonaut]

Since we don't expose the Exception interface in the public API, it's not possible to read or change error messages in a before_send handler. Exposing the whole Exception interface is a big investment, so instead we could add the following API methods:

event.get_exception_value(index: int) -> String
event.set_exception_value(index: int, value: String)
event.get_exception_count() -> int

[linear]

GODOT-124