BUG/MINOR: log: fix erroneous error message in log-steps parser

BUG/MINOR: log: fix erroneous error message in log-steps parser

When invalid arguments are provided to "log-steps" directive, the
following error is returned: "invalid arguments, expects 'all' or a
composition of logging steps separated by spaces."

While in fact the parser expects comma separated values, and even the
documentation states it.

Let's fix the error message to match with the actual behavior and doc.

It should be backported to 3.2.