Fail new tree creation when layer files exist

[poszu]

Added a check if the data dir doesn't contain any layer cache files when creating a new merkle tree.

There was a corner case that caused an invalid proof. A new merkle tree was created in a directory that contained layer cache files from a failed round recovery.

[codecov]

Codecov Report

Merging #414 (5738ecc) into develop (bc84a36) will decrease coverage by 0.1%.
The diff coverage is 62.5%.

@@            Coverage Diff            @@
##           develop    #414     +/-   ##
=========================================
- Coverage     75.1%   75.1%   -0.1%     
=========================================
  Files           23      23             
  Lines         1909    1917      +8     
=========================================
+ Hits          1435    1440      +5     
- Misses         357     359      +2     
- Partials       117     118      +1     

Files	Coverage Δ
prover/prover.go	62.8% <62.5%> (-0.1%)	⬇️