Stop model when using use_cavity=false with cavity mesh

[FinnHeu]

Raise an error and stop the model when the mesh folder contains cavity mesh files (cavity_elvls.out) but the user set use_cavity=.false. in namelist.config to prevent cavity grid cells being unintendedly treated as open ocean. #872

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  0:   ERROR: use_cavity=.false. but mesh contains cavity_elvls.out       
  0:          This mesh was prepared for cavity-resolving simulations.    
  0:          Running with use_cavity=.false. will treat cavity cells     
  0:          as open ocean, which is likely unintended.                  
  0:                                                                          
  0:          --> Either set use_cavity=.true. in namelist.config to      
  0:              enable cavities, or use a corresponding mesh            
  0:              without cavity files (e.g. CORE2 instead of CORE2ice).  
  0:  ____________________________________________________________________

[JanStreffing]

I agree this is a good first step. I think the medium term goal should be to have the model behave as expected. I.e. which setting cavity = false, the cavity areas are treated as land, not as open ocean.