Add a check that stack has been updated in CFNMain

[adanaja]

Check for UPDATE_COMPLETE after execute_change_set and exit with error code 1 if the update failed

Ref #155

[codecov]

Codecov Report

Merging #156 (8f3609e) into master (55987b4) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
- Coverage   78.33%   78.31%   -0.02%     
==========================================
  Files          43       43              
  Lines        3351     3353       +2     
==========================================
+ Hits         2625     2626       +1     
- Misses        726      727       +1     

Impacted Files	Coverage Δ
src/e3/aws/cfn/main.py	75.29% <50.00%> (-0.31%)	⬇️

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[pierretr]

A more pythonic way of returning an error would be to raise an exception here.
But that's not part of your change as the method was already returning 1 on failures.