Make check_exists cache negative results as well as positive ones

[tetron]

When loading a workflow with URL references that can't be resolved, don't keep trying them over and over again.

[codecov[bot]]

Codecov Report

Merging #580 (e3271d5) into main (b39d984) will decrease coverage by 0.40%. The diff coverage is 60.00%.

:exclamation: Current head e3271d5 differs from pull request most recent head 6dbd430. Consider uploading reports for the commit 6dbd430 to get more accurate results

@@            Coverage Diff             @@
##             main     #580      +/-   ##
==========================================
- Coverage   83.47%   83.08%   -0.40%     
==========================================
  Files          22       22              
  Lines        4502     4486      -16     
  Branches     1207     1239      +32     
==========================================
- Hits         3758     3727      -31     
- Misses        482      488       +6     
- Partials      262      271       +9     

Files Changed	Coverage Δ
schema_salad/fetcher.py	63.47% <60.00%> (-1.66%)	:arrow_down:

... and 19 files with indirect coverage changes

[tetron]

FWIW while I believe this is still a useful optimization, it turned out redundant failed lookups was not a major performance issue for the particular workflow I was working on.