API design

[Enyium]

• SingletonProcess::try_new() should just be new(), because it's not an alternative to a panicking constructor. Example from std: https://doc.rust-lang.org/std/ffi/struct.CString.html#method.new.
• unwrap_or_else() would be better than unwrap_or()
• According to the design of other popular crates like anyhow, notify, clap and windows-core, SingletonProcessError should just be named Error (and stay in the crate root).

BTW:

• The "Minimum Supported Rust Version" in your readme is outdated and doesn't match that in Cargo.toml anymore.