If you are getting the same error response from upgrading more than one fgt device then it sounds like a ticket for Fortinet support. If you have only tried it on one fgt device then it is likely a problem with that one device.
The purpose of performing a " manual" firmware upgrade is to rule out the fgr (or firmware stored on it) is the possible cause. I suggest manually downloading the 5.0.2 firmware (and validate the checksum) for one of the fgt devices and log into that device' s GUI -- perform an firmware upgrade -- see if the upgrade goes through.
But before performing a manually firmware upgrade, I suggest getting someone on site to keep/store a copy of the existing firmware/config on a USB stick in case something goes wrong -- you can get that person to restore both (if it is possible). (But be prepared for the worst.)
An ehci_hcd fatal error may be a sign of a failing flash drive. Formatting the boot device may fix it, but you may end up RMA' ing the device.
NSE4/FMG-VM64/FortiAnalyzer-VM/6.0 (FWF30E/FW92D/FGT200D/FGT101E/FGT81E)/ FAP220B/221C