MINOR: cfgparse: adapt warnif_cond_conflicts() error output

MINOR: cfgparse: adapt warnif_cond_conflicts() error output

Utility function warnif_cond_conflicts() is used when parsing an ACL.
Previously, the function directly calls ha_warning() to report an error.
Change the function so that it now takes the error message as argument.
Caller can then output it as wanted.

This change is necessary to use the function when parsing a keyword
registered as cfg_kw_list. The next patch will reuse it.