When running on HSI clock (MCCI STM32L0 devices), clock error must be set

[terrillmoore]

Some STM32L0 devices (such as my test device) won't join reliably if clock error is zero; the HSI clock is too far off. Until we switch to using the 32kHz crystal, we must set a clock error by default in CatenaStm32L0::LoRaWAN::begin().