fix(cli) correct error message on missing Serf
fix(cli) correct error message on missing Serf

Generating the error message concatenated a table by omitting
a `tostring()` call.

Fix #2217
jebenexer/kong,shiprabehera/kong,li-wl/kong,salazar/kong,Kong/kong,Kong/kong,Mashape/kong,Kong/kong,akh00/kong,icyxp/kong