LXC.run_command: handle enoexec and enoent

Author: aither64

ext/lxc/lxc.c

@@ -109,7 +109,7 @@ lxc_run_command(VALUE self, VALUE rb_command)
  cmd.argv = ruby_to_c_string_array(rb_command);
 
  ret = lxc_attach_run_command(&cmd);
- if (ret == -1)
+ if (ret == -1 || ret == 126 || ret == 127)
  rb_raise(Error, "unable to run command on attached container");
  /* NOTREACHED */
  return Qnil;