[hotfix] catch OOM error in `Manager.loadEnvelop`

• context: experiencing bugs on startup that cause numbers to become completely unresponsive
• likely cause:
  • Manager.loadEnvelop tries to allocate byteArray of contentLen
  • but contentLen is ridiculously large b/c file was garbage, causing in.readInt() to return nonsense value
  • :. when system tries to allocate byte array, it runs out of heap space
• attempted fix:
  • catch the OOM and return null (which is handled gracefully by caller in `Manager.retryFailedMessages``