meta/lib/oe/sstatesig.py: do not error out if sstate files fail on os.stat()

meta/lib/oe/sstatesig.py: do not error out if sstate files fail on os.stat()

There's an ongoing issue with the autobuilder NFS:
https://autobuilder.yoctoproject.org/typhoon/#/builders/87/builds/6463/steps/14/logs/stdio

The file entry exists, but os.stat returns a 'file not found; error. It's not
clear how and why such entries appear, but they do produce printdiff test failures
and should not be relevant in context of the printdiff.

[RP: Move wrapping to get_time() function to cover all cases and add comment]
Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>