key_manager: Don't assume file seeks and reads will always succeed

Given the filesystem should always be assumed to be volatile, we should
check and bail out if a seek operation isn't successful. This'll prevent
potentially writing/returning garbage data from the function in rare
cases.

This also allows removing a check to see if an offset is within the
bounds of a file before perfoming a seek operation. If a seek is
attempted beyond the end of a file, it will fail, so this essentially
combines two checks into one in one place.