"E1"/"E2" error messages when increasing antenna gain above default

[andreas-mucha]

When adjust the antenna gain above the default value of 4 using the RFID.set_antenna_gain() method introduced in #32, e.g. in a slightly modified Read.py example, sequences of "E1" and "E2" are printed. They seem to originate from the RFID.card_write() method, however the code there is a bit opaque to me.