sanity: Add error check for '%' in build path

It has been reported that '%' characters in build paths break with python
exceptions, probably due to confusion with python string escaping. Whilst it
is probably fixable, showing the user a human readable error is better given
it doesn't work.

[YOCTO #14282]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>