implement proper failed error to distinguish between not supported and other kind of failure

[Murmele]

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

[dhardy]

Distinguishing Failed and NotSupported is potentially confusing. I suggest instead this:

#[non_exhaustive]
pub enum NotSupportedError {
    Transparency,
    PopupWindow,
    // ...
    // If required (but preferably not), a generic option:
    Other(NotSupportedReason),
}

A possible variant of the above would be to add a "reason" payload to each variant:

pub struct ErrorDescription;

#[non_exhaustive]
pub enum NotSupportedError {
    Transparency(ErrorDescription),
    PopupWindow(ErrorDescription),
    // ...
    // If required (but preferably not), a generic option:
    Other(ErrorDescription),
}

Motivation: (a) named variants make it is clear (from a documentation perspective) what the cause of the error was (e.g. Transparency is not supported) and (b) user code can more easily (and less confusingly) react to such errors (e.g. "we must disable transparency" or "we must emulate pop-up windows").

[Murmele]

Thanks for your PR. I will close this in favour of your one: #4707