lib/sstatesig/find_siginfo: raise an error instead of returning None when obtaining...

lib/sstatesig/find_siginfo: raise an error instead of returning None when obtaining mtime

Suppressing the error and returning None can result in a delayed failure:
https://autobuilder.yoctoproject.org/typhoon/#/builders/86/builds/6254/steps/14/logs/stdio
https://autobuilder.yoctoproject.org/typhoon/#/builders/87/builds/6262/steps/14/logs/stdio

It is not clear why the os.stat() error occurs to begin with (it shouldn't), so rather than
adding further workarounds, let's get diagnostics at the source first, so we understand
what is going on.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>