classes/buildstats: don't catch all errors from mkdirhier

classes/buildstats: don't catch all errors from mkdirhier

mkdirhier() doesn't raise an exception if the directory exists, so if it
does raise one we should just be failing, otherwise we're just going
to hit errors later anyway.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>