Add error message for not collected tasks with @task decorator.

[tobiasraabe]

Closes #513.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (88a5fb1) 96.96% compared to head (96a337b) 96.96%.

Files	Patch %	Lines
src/_pytask/collect.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #521      +/-   ##
==========================================
- Coverage   96.96%   96.96%   -0.01%     
==========================================
  Files         104      104              
  Lines        8974     8997      +23     
==========================================
+ Hits         8702     8724      +22     
- Misses        272      273       +1     

Flag	Coverage Δ
end_to_end	82.13% <40.00%> (-0.12%)	⬇️
integration	40.06% <16.00%> (-0.10%)	⬇️
unit	68.84% <88.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.