rust-agent: Remove uses of deprecated functions

This addresses the following:

    warning: use of deprecated item 'std::error::Error::description': use the Display impl or to_string()
        --> rustjail/src/container.rs:1598:31
         |
    1598 | ...                   e.description(),
         |                         ^^^^^^^^^^^
         |
         = note: `#[warn(deprecated)]` on by default

Fixes: #750

Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>

src/agent/rustjail/src/container.rs

@@ -1545,7 +1545,7 @@ fn execute_hook(logger: &Logger, h: &Hook, st: &OCIState) -> Result<()> {
                         info!(
                             logger,
                             "wait child error: {} {}",
-                            e.description(),
+                            e,
                             e.raw_os_error().unwrap()
                         );