Improve error message in ossec-testrule for missing rule matches

[atomicturtle]

Changed cryptic error 'currently_rule not set!' to a more helpful message that explains what's actually wrong and how to fix it.

Old message:
'ossec-testrule: currently_rule not set!'

New message:
'ossec-testrule: No rule matched for decoder '<decoder_name>'.
Verify that rules exist for this decoder and check if rules have
required dependencies (if_sid, if_matched_sid, if_matched_group, etc.).'

This helps users quickly identify that:

1. The decoder matched but no rule matched
2. They should check if rules exist for this decoder
3. Rules may be missing required dependencies like if_sid

Fixes issue #2093 reported by stefanct.