event: Fix subprocess event error traceback failures

event: Fix subprocess event error traceback failures

If subprocess raises a CalledProcessError() error, e.g. from a call
like subprocess.check_call("false"), bitbake would try and pass the
object over IPC and fail, leading to an unusual error:

('__init__() takes at least 3 arguments (1 given)', <class 'subprocess.CalledProcessError'>, ())%

To avoid this, we turn the value into a string which prevents the
issues the IPC has trying to deal with the object (for the same reason
we deal with tracebacks here too).

[YOCTO #8752]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>