px_uploader: fix SerialException error
px_uploader: fix SerialException error

Sometimes right after reboot, we got a `raise SerialException(
msg.errno, "could not open port {}: {}".format(self._port, msg))`.
If this happens now, we will just try again later.

PX4/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,krbeverx/Firmware,dagar/Firmware,dagar/Firmware,PX4/Firmware,jlecoeur/Firmware,acfloria/Firmware,dagar/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,Aerotenna/Firmware,krbeverx/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,dagar/Firmware,Aerotenna/Firmware,PX4/Firmware,dagar/Firmware,jlecoeur/Firmware,acfloria/Firmware,krbeverx/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,jlecoeur/Firmware,acfloria/Firmware,mcgill-robotics/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,dagar/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,mcgill-robotics/Firmware