In i3 the ipc reply will contain a human readable error message, and
this patch replicates that behaviour.
However, that error message is also useful for logging, which this
patch takes advantage of.
E.g. instead of logging errors directly in commands.c/checkargs, it is
fed back to the caller which eventually ends up logging everything with
maximum context available (config.c/read_config).
So instead of logging e.g. "Error on line 'exit'" it will now log:
"Error on line 'exit': Can't execute from config."
