set_selinux_security_context: also display the error from the crate

+ fix comments from review

Author: sylvestre

src/uu/cp/src/cp.rs

@@ -2458,7 +2458,11 @@ fn copy_file(
     #[cfg(feature = "selinux")]
     if options.set_selinux_context && uucore::selinux::is_selinux_enabled() {
         // Set the given selinux permissions on the copied file.
-        uucore::selinux::set_selinux_security_context(dest, options.context.as_ref())?;
+        if let Err(e) =
+            uucore::selinux::set_selinux_security_context(dest, options.context.as_ref())
+        {
+            return Err(Error::Error(format!("SELinux error: {}", e)));
+        }
     }
 
     copied_files.insert(