properly handle bad config and fail early

[deadc0de6]

for #443

[codecov]

Codecov Report

Attention: Patch coverage is 45.45455% with 12 lines in your changes missing coverage. Please review.

Project coverage is 91.72%. Comparing base (ef88de7) to head (77b7045).
Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
dotdrop/cfg_yaml.py	40.00%	9 Missing ⚠️
dotdrop/dotdrop.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #445      +/-   ##
==========================================
+ Coverage   90.97%   91.72%   +0.74%     
==========================================
  Files          22       23       +1     
  Lines        4753     5073     +320     
==========================================
+ Hits         4324     4653     +329     
+ Misses        429      420       -9     

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