raise if Singularity container retrieval fails

[fabianegli]

There has been a test that xfailed - It seems that failing to retrieve a container image should have always raised an exception, but it didn't. Now it does and the test that was maked xfail is now split into two tests, one that tests the failure mode if Singularity isn't installed and one for the case Singularity is installed but the container image retrieval fails.

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 #1622 (8b751ef) into dev (dd7b59b) will decrease coverage by 0.06%.
The diff coverage is 10.00%.

@@            Coverage Diff             @@
##              dev    #1622      +/-   ##
==========================================
- Coverage   64.44%   64.37%   -0.07%     
==========================================
  Files          54       54              
  Lines        6288     6294       +6     
==========================================
  Hits         4052     4052              
- Misses       2236     2242       +6     

Impacted Files	Coverage Δ
nf_core/download.py	53.69% <10.00%> (-0.79%)	⬇️

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[mirpedrol]

I think it's worth adding this change to CHANGELOG, otherwise it looks good to me