remove imprecise error handling in PhotoTour dataset

[pmeier]

Addresses #3448 (comment). .cache() will raise a FileNotFoundError and thus there is no need to process this in any way.

[codecov]

Codecov Report

Merging #3488 (4248584) into master (7992eb5) will decrease coverage by 0.55%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3488      +/-   ##
==========================================
- Coverage   79.26%   78.70%   -0.56%     
==========================================
  Files         105      105              
  Lines        9742     9708      -34     
  Branches     1564     1555       -9     
==========================================
- Hits         7722     7641      -81     
- Misses       1533     1576      +43     
- Partials      487      491       +4     

Impacted Files	Coverage Δ
torchvision/datasets/phototour.py	81.63% <100.00%> (-0.55%)	⬇️
torchvision/datasets/flickr.py	25.64% <0.00%> (-73.08%)	⬇️
torchvision/ops/boxes.py	87.77% <0.00%> (-3.49%)	⬇️
torchvision/transforms/functional_tensor.py	79.19% <0.00%> (-0.66%)	⬇️
torchvision/models/detection/_utils.py	82.58% <0.00%> (-0.53%)	⬇️
torchvision/transforms/functional_pil.py	67.61% <0.00%> (-0.52%)	⬇️
torchvision/transforms/autoaugment.py	94.04% <0.00%> (-0.08%)	⬇️
torchvision/transforms/transforms.py	84.28% <0.00%> (-0.03%)	⬇️
torchvision/transforms/functional.py	81.77% <0.00%> (ø)
torchvision/datasets/utils.py	72.58% <0.00%> (+2.37%)	⬆️
... and 3 more

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[fmassa]

Thanks!