Check RFM9x version and more verbose errors

• Incorrectly setting tx_power and frequency_mhz throws errors describing the accepted range
• RFM9x now checks version to verify correct wiring

Thanks @terriko!

To use in CircuitPython, simply install the Adafruit CircuitPython bundle.

To use in CPython, pip install adafruit-circuitpython-rfm9x.

Read the docs for info on how to use it.