Add debugger visualizer for anyhow::Error

[dtolnay]

anyhow::Result looks like this in LLDB:

(core::result::Result<(), anyhow::Error>) {
  value = {
    0 = {
      inner = {
        ptr = {
          pointer = 0x00007f8008414040
        }
      }
    }
  }
}

We can wire up something better with https://doc.rust-lang.org/1.82.0/reference/attributes/debugger.html#using-debugger_visualizer-with-gdb.