adds error when storyshots finds no stories

[theshortcut]

Issue: #1330

What I did

Added an error when storyshots finds no stories. This is makes the test suite failure more obvious than the current failure of no tests being run.

How to test

Modify .storybook/config.js to find no stories, or create a new project with a storyshots suite without any stories.

Is this testable with jest or storyshots?
no
Does this need a new example in the kitchen sink apps?
no
Does this need an update to the documentation?
no
If your answer is yes to any of these, please make sure to include it in your PR.

[Hypnosphi]

Totally makes sense to me, thanks!

[codecov]

Codecov Report

Merging #2079 into release/3.3 will increase coverage by <.01%.
The diff coverage is 50%.

@@               Coverage Diff               @@
##           release/3.3    #2079      +/-   ##
===============================================
+ Coverage        21.71%   21.72%   +<.01%     
===============================================
  Files              355      355              
  Lines             7059     7061       +2     
  Branches           909      910       +1     
===============================================
+ Hits              1533     1534       +1     
- Misses            4839     4855      +16     
+ Partials           687      672      -15

Impacted Files	Coverage Δ
addons/storyshots/src/index.js	79.59% <50%> (-1.26%)	⬇️
app/vue/src/server/config/babel.js	0% <0%> (-100%)	⬇️
lib/ui/src/modules/ui/libs/hierarchy.js	50.74% <0%> (ø)	⬆️
...res__/update-addon-info/update-addon-info.input.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Number.js	8.06% <0%> (ø)	⬆️
addons/info/src/components/types/OneOfType.js	11.11% <0%> (ø)	⬆️
lib/components/src/highlight_button.js	0% <0%> (ø)	⬆️
...es__/update-addon-info/update-addon-info.output.js	0% <0%> (ø)	⬆️
addons/viewport/src/components/RotateViewport.js	22.72% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/usplit.js	38.7% <0%> (ø)	⬆️
... and 27 more

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[Hypnosphi]

@ndelangen @shilman technically it's a feature, so should we rebase it on relase/3.3?

[ndelangen]

This will introduce a breaking build when there wasn't one before. Definitely a 3.3 feature!

[theshortcut]

What is the new build failure? A build without any stories will already currently fail due to Jest failing a suite with zero tests.

[ndelangen]

@theshortcut ow, right. You're 💯% correct!