RawUnexpected constructor names

[botahamec]

The two constructors for RawUnexpected are:

• RawUnexpected::new: takes an Error
• RawUnexpected::msg: takes a Display + Debug

I'm wondering if we'd like to change them to from_err and from_msg respectively, following the Rust API Guidelines. I used new and msg because that's what anyhow and eyre use.