CLI: handle promise rejections

[Hypnosphi]

Issue: promise rejections are left unhandled, which leads to node warning and false-negative tests

What I did

How to test

• add this line to const end handler in generate.js:

  throw new Error("OOPS");
  

• run CLI test: yarn test --cli

Before:

 • Adding storybook support to your "Meteor" app. ✓
(node:48487) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: OOPS
(node:48487) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

 getstorybook - the simplest way to add a storybook to your project. 

 • Detecting project type. ✓
 • Adding storybook support to your "React" app

Note that tests keep running without error

After:

 • Adding storybook support to your "Meteor" app. ✓
Error: OOPS
    at end (/Users/jetbrains/IdeaProjects/storybook/lib/cli/bin/generate.js:50:9)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)
npm ERR! Test failed.  See above for more details.
error Command failed with exit code 1.

[codecov]

Codecov Report

Merging #1826 into release/3.3 will decrease coverage by 0.01%.
The diff coverage is 0%.

@@               Coverage Diff               @@
##           release/3.3    #1826      +/-   ##
===============================================
- Coverage        22.46%   22.45%   -0.02%     
===============================================
  Files              321      321              
  Lines             6276     6279       +3     
  Branches           787      793       +6     
===============================================
  Hits              1410     1410              
+ Misses            4275     4261      -14     
- Partials           591      608      +17

Impacted Files	Coverage Δ
lib/cli/bin/generate.js	0% <0%> (ø)	⬆️
addons/info/src/components/Node.js	39.43% <0%> (ø)	⬆️
lib/ui/src/modules/api/actions/api.js	50.47% <0%> (ø)	⬆️
addons/knobs/src/react/WrapStory.js	12% <0%> (ø)	⬆️
.../src/modules/ui/components/stories_panel/header.js	29.62% <0%> (ø)	⬆️
app/react/src/client/preview/element_check.js	41.17% <0%> (ø)	⬆️
addons/info/src/components/types/OneOfType.js	11.11% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_keyevents.js	33.33% <0%> (ø)	⬆️
lib/codemod/src/transforms/update-addon-info.js	51.16% <0%> (ø)	⬆️
addons/info/src/components/markdown/htags.js	30% <0%> (ø)	⬆️
... and 30 more

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