Add must_use to Error constructors

[dtolnay]

This catches the following bug:

use anyhow::anyhow;

fn main() -> anyhow::Result<()> {
    if true {
        // meant to write bail!
        anyhow!("it failed");
    }
    Ok(())
}

warning: unused return value of `must_use` that must be used
 --> tests/ui/must-use.rs:8:9
  |
8 |         anyhow!("it failed");
  |         ^^^^^^^^^^^^^^^^^^^^