Catch FileNotFoundError when looking for TODO comments

[ewels]

Fixes #796
Also switched the log message from error to critical when halting linting due to failed tests.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #804 (80e6fdd) into dev (8ae626a) will decrease coverage by 0.05%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##              dev     #804      +/-   ##
==========================================
- Coverage   82.79%   82.73%   -0.06%     
==========================================
  Files          13       13              
  Lines        2702     2705       +3     
==========================================
+ Hits         2237     2238       +1     
- Misses        465      467       +2     

Impacted Files	Coverage Δ
nf_core/lint.py	90.95% <61.53%> (-0.28%)	⬇️

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