Blocks: Capture and recover from block rendering runtime errors

[aduth]

This pull request seeks to add error handling to block edit rendering. Error boundaries are a new feature added in React 16 which add a simple componentDidCatch lifecycle hook to capture errors which occur during a component's rendering. With these changes, an error boundary wraps rendering of each block and captures errors, presenting a warning message if one occurs, rather than allowing it to go unhandled and crash the entire application.

Before	After

Implementation notes:

The original implementation attempt applied componentDidCatch to VisualEditorBlock itself but this had some odd effects on componentDidMount and refs, so it was later revised to create a separate wrapping error boundary component.

Testing instructions:

1. Navigate to Gutenberg > Demo, or Gutenberg > New Post
2. In your browser developer tools console, register the Mr. Crashy block:
   • wp.blocks.registerBlockType( 'myplugin/mr-crashy', { title: 'Mr. Crashy', category: 'common', icon: 'warning', edit() { throw new Error(); }, save() { return ''; } } );
3. From the inserter menu, select Mr. Crashy
4. Note that the block is inserted and displays a warning about failure

Open questions:

• What do we do about saved content? If an error occurs after making some revisions, the changed block attributes are still reflected in the saved output. To me, this seems reasonable, but is that expectation shared?
• How can we surface the actual error message that was captured? Might be better to be addressed in a subsequent pull request.

[codecov]

Codecov Report

Merging #2267 into master will decrease coverage by 0.1%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #2267      +/-   ##
=========================================
- Coverage    23.9%   23.8%   -0.11%     
=========================================
  Files         144     147       +3     
  Lines        4530    4550      +20     
  Branches      766     771       +5     
=========================================
  Hits         1083    1083              
- Misses       2912    2927      +15     
- Partials      535     540       +5

Impacted Files	Coverage Δ
editor/modes/visual-editor/block-crash-warning.js	0% <0%> (ø)
editor/modes/visual-editor/create-block-warning.js	0% <0%> (ø)
editor/modes/visual-editor/block.js	0% <0%> (ø)	⬆️
editor/modes/visual-editor/block-crash-boundary.js	0% <0%> (ø)
...ditor/modes/visual-editor/invalid-block-warning.js	0% <0%> (ø)	⬆️

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[youknowriad]

This is a really awesome React Feature.
it works great.

I also find it reasonable to continue running the save function like any other block. An alternative would be to return the originally loaded markup.