better logging if directory not correctly specified

[KevinMenden]

Add a better logging message if a directory has not specified correctly.

closes #940

[codecov]

Codecov Report

Merging #942 (b3c1d93) into dev (c51f8f0) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

❗ Current head b3c1d93 differs from pull request most recent head 655a1cf. Consider uploading reports for the commit 655a1cf to get more accurate results

@@            Coverage Diff             @@
##              dev     #942      +/-   ##
==========================================
- Coverage   69.59%   69.56%   -0.03%     
==========================================
  Files          34       34              
  Lines        4173     4173              
==========================================
- Hits         2904     2903       -1     
- Misses       1269     1270       +1     

Impacted Files	Coverage Δ
nf_core/modules/create.py	60.46% <0.00%> (ø)
nf_core/__main__.py	61.26% <0.00%> (-0.32%)	⬇️

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[ewels]

Nice!