LMIC should not assert()

[terrillmoore]

assert()s are generally harmful, particularly as they cause a device hang in this implementation (rather than a restart) -- you'll have to roll a truck to get the device going again.

A code review is needed so that places where we currently assert instead:

• return errors passed back to the caller,
• continue by substituting sane values, or
• shut down the LMIC code (potentially reinitializing later)