Improved error checking in global addDecorator

[mattleff]

Throw on adding a global decorator after stories have already been added. The global decorator will not be applied.

Issue: #877, #879

What I did

Modified the global addDecorator to throw if stories have already been added. This helps avoid issues like #877 and #879.

How to test

yarn test

[codecov]

Codecov Report

Merging #1481 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1481      +/-   ##
==========================================
+ Coverage   14.54%   14.61%   +0.07%     
==========================================
  Files         202      202              
  Lines        4649     4653       +4     
  Branches      494      501       +7     
==========================================
+ Hits          676      680       +4     
+ Misses       3554     3539      -15     
- Partials      419      434      +15

Impacted Files	Coverage Δ
app/react/src/client/preview/client_api.js	42.04% <100%> (+2.75%)	⬆️
...react-native/src/manager/components/PreviewHelp.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_keyevents.js	33.33% <0%> (ø)	⬆️
addons/knobs/src/components/types/Select.js	7.81% <0%> (ø)	⬆️
addons/info/src/components/PropVal.js	0% <0%> (ø)	⬆️
app/react/src/server/config.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/config_api.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/usplit.js	38.7% <0%> (ø)	⬆️
app/react/src/client/preview/reducer.js	0% <0%> (ø)	⬆️
.../ui/src/modules/ui/components/left_panel/header.js	28.57% <0%> (ø)	⬆️
... and 24 more

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[shilman]

Awesome! Thanks for getting this fixed up! 🎖