Crash if path specified in `SOLAR_DIR` in config file does not exist

[iingram]

One solution could be to wrap the place(s) where this path is used in a try/except, where except block logs a usefully worded error and also, maybe more importantly, writes something out to the user warning them.

Beyond checking that the path exists, may also want to check that the user that the firmware is running under has permissions to write there.