varlinkctl: when operating in --more mode, fail correcly on Varlink method error

varlinkctl: when operating in --more mode, fail correcly on Varlink method error

In varlink.c we generally do not make failing callback functions fatal,
since that should be up to the app. Hence, in case of varlinkctl (where
we want failures to be fatal), make sure to propagate the error back
explicitly.

Before this change a failing call to "varlinkctl --more call …" would result in
a zero exit code. With this it will correctly exit with a non-zero exit
code.