sstatesig: be more precise and show the full path in exceptions

sstatesig: be more precise and show the full path in exceptions

Also remove the warning than doesn't make sense as the code will generate
an exception and bitbake will abort.

Before:
| WARNING: core-image-minimal-initramfs-1.0-r0 do_image_complete: KeyError in .
| Exception: Exception: KeyError: 'getpwuid(): uid not found: xxxxx'
| Path . is owned by uid xxxxx, gid yyy, which doesn't match any user/group on target. This may be due to host contamination.

After:
| Exception: Exception: KeyError: 'getpwuid(): uid not found: xxxxx'
| Path /build/tmp/work/intel_corei7_64-lmp-linux/core-image-minimal-initramfs/1.0-r0/sstate-build-image_complete is owned by uid xxxxx, gid yyy, which doesn't match any user/group on target. This may be due to host contamination.

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>