Check errors with derive macro attributes

**Bug description**

The `CompositeTemplate` macro (and probably others too) currently do not sufficiently check the validity of present attributes. So instead of getting a compile error, things will silently fail.

```rust
#[derive(CompositeTemplate, Default)]
#[template(file = "../../res/window.ui")]
pub struct CustomWidget {
	#[template]
    header: adw::HeaderBar,
}
```