agent: Make wording of error message match CRI-O test suite

The CRI-O integration test suite has two tests that fail because they search for
"not found" in the error message, but we emit "is not exist".

Change the error message to match the expectations of the test suite.

Fixes: kata-containers#2036

Reported-by: Julien Ropé <jrope@redhat.com>
Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>

src/agent/rustjail/src/container.rs

@@ -641,7 +641,7 @@ fn do_init_child(cwfd: RawFd) -> Result<()> {
     let exec_file = Path::new(&args[0]);
     log_child!(cfd_log, "process command: {:?}", &args);
     if !exec_file.exists() {
-        find_file(exec_file).ok_or_else(|| anyhow!("the file {} is not exist", &args[0]))?;
+        find_file(exec_file).ok_or_else(|| anyhow!("the file {} was not found", &args[0]))?;
     }
 
     // notify parent that the child's ready to start