Hello,
There are alarms for PoE power supply failure as well:
You can find here all the PoE alarms available: AR550 Poe Alarms
This alarm indicates that the consumption is bigger than the supply (in case of Power supply failure this will occur) PWRUTITHRESH
And here you can find all the MIB objects that can be followed regarding PoE: AR550 PoE MIB
This OID indicates that not enough power is given by PoE PSU: hwPoePSEPowerCut
Please let me know if this helps and if there is anything else I can assist with you.