Fix: tools: crm_verify distinguishes configuration warnings and errors

The output of crm_verify no longer distinguished configuration warnings
from errors as brought up from:
https://github.com/ClusterLabs/pacemaker/pull/3266#discussion_r1636411072

Regression introduced in 2.1.7 by 0c5a32624a

This commit fixes it by splitting output_config_issue() into separate
error and warning handlers, and add the "warning:" or "error:" prefix to
the message.

Fixes T834