Remove calls to unwrap

[vbrandl]

(From my comment on reddit and as a reminder for myself)
You might want to change the notify method to return a Result<(), SomeErrorType> and don't call unwrap() in your code. It is considered bad practice for a library to panic.

[frewsxcv]

I started working on this (here's the branch), but a little blocked on moving forward because the winrt crate errors don't implement Error

[vbrandl]

I missed your comment here and implemented it already.

I implemented a new Error type for rust-notifica, which wraps the external error types. I could not test it on Windows, but maybe you can.

[frewsxcv]

reopening – we still have a few unwrap calls we need to remove

[Absolucy]

These two unwraps WILL panic on macOS if notify is used more than once: https://github.com/frewsxcv/rust-notifica/blob/master/src/lib.rs#L177-L178

set_application should be run within a std::sync::Once.