entrypoint returns boolean

This is a minor change to the C Extension API entrypoint: instead of returning void and relying on an extension calling extension_access->set_error, we now require extensions to explicitly return true indicating the initialization process was successful. This makes things a bit more pleasant during development because DuckDB will then throw if somehow the initialization process returned early but failed to set an error message:

FATAL Error: Extension 'demo_capi' failed to initialize but did not return an error. This indicates an error in the extension: C API extensions should return a boolean `true` to indicate succesful initialization. This means that the Extension may be partially intialized resulting in an inconsistent state of DuckDB.

extension_access->set_error can take nullptr

Extensions can now call extension_access->set_error with a nullptr for the error message. DuckDB will then just throw a generic error:

Error: An error was thrown during initialization of the extension 'demo_capi': Extension has indicated an error occured during initialization, but did not set an error message.