tools: unbless errors in run_fork_with_timeout

tools: unbless errors in run_fork_with_timeout

We cannot properly encode blessed objects as json, so
instead, we should stringify them. This happened for
instance if a VM's systemd scope wasn't cleaned up as we
got an error as a Net::DBus::Error object causing a
"malformed json string" error to appear instead of the
actual message.

Additionally, add a 'must_stringify' helper: The above error
object implements a '""' operator for stringification (as
all error should), but in theory that could die as well, in
which case we just return a generic error string we'll
hopefully never see...

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>