Replace unwraps with expects in bevy_window, bevy_scene and bevy_diagnostics

[druskus20]

Objective

• Make bevy's errors more descriptive and easier to understand
• Related: Replace unwraps with expects in bevy_app, bevy_audio, bevy_core, and bevy_core_pipeline #3913
• Tracking issue: Tracking issue: Replace unwrap with expect #3899

Solution

• Replace unwrap with expect in some of the crates
• turn ComponentDescriptor::type_id into a TypeId (from Option)

[alice-i-cecile]

Thanks! This is a solid start, and definitely an improvement over the status quo. I left some comments; the type registry error needs some digging, and we should make the errors for missing resources more helpful.

[alice-i-cecile]

FYI, you can "Mark comment as resolved", which helps keep the PR tidy and easy to follow for reviewers :)

[alice-i-cecile]

Closing per discussion in #3899 <3