Fix possible UR, clear compile warnings, add printPalErrorString() wrapper

[felix-kolbe]

I improved the library regarding following points:

• Fix a possible undefined read error in VL53L0X_CheckAndLoadInterruptSettings().
• Clean library from compiler warnings by removing unused variables.
• Add Adafruit_VL53L0X::printPalErrorString() to human-readably print the status code returned e.g. by Adafruit_VL53L0X::getSingleRangingMeasurement().

Thanks for maintaining and merging :-)