kernel-yocto: improve fatal error messages of symbol_why.py

Improve the fatal error message of the yocto-kernel-tools symbol_why.py
and shows the command that generate the error as it can help understand
the root cause of the error.

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>