Catch AssertionError from schema build

[ewels]

Fixes bug found by @erikrikarddaniel where error from nextflow config raised an AssertionError that was not handled.

Untested.

[ewels]

To replicate error, introduce a syntax error in nextflow.config etc. Without this fix you should get a big traceback. With the fix you should get a nicely formatted message that prints the nextflow error message.

[ewels]

Something fishy going wrong with the Black GitHub action - seems to be an installation error with Black, nothing related to the code change.

[ewels]

Hopefully this PR should fix the actions error: #1268

[codecov]

Codecov Report

Merging #1267 (a135b8e) into dev (f58077e) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              dev    #1267      +/-   ##
==========================================
+ Coverage   67.52%   67.54%   +0.01%     
==========================================
  Files          50       50              
  Lines        5611     5611              
==========================================
+ Hits         3789     3790       +1     
+ Misses       1822     1821       -1     

Impacted Files	Coverage Δ
nf_core/__main__.py	55.60% <0.00%> (ø)
nf_core/utils.py	84.50% <ø> (+0.25%)	⬆️

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[ewels]

Tested locally and seems to be working well ✅