Incorrect debug log output when actions are suppressed while maintenance period

Not suitable log with DebugLevel=4 appears when actions are suppressed in when a maintenance period.
Because the case of ZBX_ESCALATION_SUPPRESS is not implemented in check_escalation_result_string().
Please see the attached patch.

Steps to reproduce:

1. Set DebugLeve=4
2. Create a maintenance period with data collection.
3. Create a series of item,trigger and  action with option "Pause operations for suppressed problems" enabled.
4. Make an event which executes the action while the maintenance period.
5. Find out the log.

Result:
 30152:20190702:174302.811 In check_escalation() escalationid:34
status:sleep
 30152:20190702:174302.811 End of check_escalation():unknown error:''

 Expected:
 14053:20190703:145402.846 In check_escalation() escalationid:1
status:sleep
 14053:20190703:145402.846 End of check_escalation():suppress error:''