sstate: don't silently handle all exceptions in sstate_checkhashes

If checkstatus returns an exception we should silently handle
FetchError, as this means the fetch failed for 'normal' reasons such as
file not found.  However, other exceptions may be raised, and these
should be made visible.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>