Serializer: Capture and recover from save errors

[aduth]

Related: #2267

This pull request seeks to appropriately recover from unhandled errors occurring during a block's save implementation. Not only does this help prevent a BSOD (blank screen of death), it also preserves the original content of the block in which the error had occurred (same treatment as in an invalid block).

Implementation notes:

The majority of changes are regenerated fixtures, because we now always assign originalContent into the parsed block object, not just when the post is invalid, since we fall back on this content if an error occurs during the save step.

Testing instructions:

You'll need to manually add the Mr. Crashy block registration prior to editor initialization. I recommend at the bottom of editor/index.js:

wp.blocks.registerBlockType( 'myplugin/mr-crashy', { title: 'Mr. Crashy', category: 'common', icon: 'warning', edit() { return ''; }, save() { throw new Error(); } } );

1. Navigate to Gutenberg > New Post
2. Add a paragraph block with some text
3. Add a Mr. Crashy block
4. Save the post
5. Refresh the page
6. Note that the text block saved correctly, and the Mr. Crashy block displays with an error message
7. More interestingly, if you manually add content between the Mr. Crashy block delimiters (in Text mode) and save the post, note that content is preserved on refreshing and later saving the post again

[codecov]

Codecov Report

Merging #2618 into master will decrease coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2618      +/-   ##
==========================================
- Coverage   33.16%   33.15%   -0.01%     
==========================================
  Files         182      182              
  Lines        5530     5540      +10     
  Branches      963      963              
==========================================
+ Hits         1834     1837       +3     
- Misses       3130     3136       +6     
- Partials      566      567       +1

Impacted Files	Coverage Δ
editor/modes/visual-editor/block-crash-boundary.js	0% <0%> (ø)	⬆️
editor/modes/visual-editor/block.js	0% <0%> (ø)	⬆️
blocks/api/serializer.js	97.91% <100%> (+0.04%)	⬆️
blocks/api/validation.js	95.16% <100%> (+0.24%)	⬆️
blocks/api/parser.js	98.14% <100%> (-0.04%)	⬇️

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