fdtgrep: Improve error handling with invalid device tree

This tool requires that the aliases node be the first node in the tree. But
when it is not, it does not handle things gracefully. In fact it crashes.

Fix this, and add a more helpful error message.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Masahiro Yamada <yamada.masahiro@socionext.com>