Improve `candle_core::Error` to make it more ergonomic

[EricLBuehler]

Currently, our Error type is already useful but could benefit from some of the QoL features which the anyhow crate has.

This PR adds the following:

• Make Error::msg more in line with anyhow::Error::msg. Although the type bound changes, this is not a breaking change because std::error::Error: Display already.
• Add the Context trait. Inspired by anyhow's Context trait which allows for .with_context and .context on Options and Results. Very useful!