Bug#973910: gpsprune: Missing tile cache root

[debian at activityworkshop.net]

Noted and understood, thanks for reporting this.

Yes I agree the reaction to this situation isn't ideal.  When the whole 
cache root goes missing it knows it _should_ use the disk cache but 
every attempt to use it silently fails.

This will be improved with 20.1.  Until then the best solution is to use 
the Settings -> Save maps to disk function, if you know that you need 
to.