sstatesig: Add descriptive error message to getpwuid/getgrgid "uid/gid not found" KeyError

If path is not owned by any user installed on target it gives
insufficient error "getpwuid(): uid not found" which may be misleading.
This exception occurs if uid/gid of path was not found in PSEUDO_PASSWD
files, which simply means the path is owned by host user and there is
host user contamination.

Add more information to the exception message to make it easier for user
to debug.

[YOCTO #14031]

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>