Epic: Error messages + linting

[cscheid]

We could spend some time improving our error messages. (This statement is always true, but there are a number of specific things we could focus on).

• Offer better error messages for malformed extensions #1570
• nicer quarto preview warnings from Lua #4782
• Inline execution: error messages are not great #8723
• difficult error with typo in var #8765
• Misguided message "Unable to locate an installed version of R" #6929
• Emit better error message in the presence of incorrect -labels specs #4938
• Warn or error if both _quarto.yml and _quarto.yaml are found #5397
• Improve error messages surrounding notebooks #8922
• nicer quarto preview warnings from Lua #4782
• Improve linting to show that out-width and out-height are being ignored in revealjs when stretch is activated (globally or per slide) #9512
• lang argument in quarto chapter yaml header is not taken into account #8953

Validation

• Use "image" schema #1794
• our YAML validator doesn't work well with YAML references #8762

Linting

• Bad shortcode syntax on a page breaks variable substitution #7975
• Warn when the same cross reference is defined more than once #7552
• Including qmd that uses jupyter, yaml top matter gets dumped into render doc sometimes #7157
• Having a section with underlined text together with unnumber operator gives error #6962
• HTML: Sections rendered within Callouts containing a desginated title will increment section-numbering in the TOC, but do not show on the TOC itself, resulting in gaps therein #6826
• jupyter field in YAML header depends on which level it is placed? #6594
• Linter: check remote sources #6328
• Lint: warn users about headers in divs in revealjs (was: Odd behavior in Print View) #5978
• lint: warn about metadata block in diagrams being in the wrong place #5767
• Warn users about crossref entries in inline images #5571
• Reconciling #| eval: !expr behaviour with knitr error handling  #5182
• YAML validation: warn about about keys with empty objects #4751
• Bug in block-style latex if exporting to docx #4740
• Capitals in Figure label strings break rendering #4542
• margin figures errors in pdf when generating plots #4011
• Document fenced div attribute syntax #3900
• yaml options are ignored if too many empty new lines are added #3560
• Error parsing --- without whitespace #3004
• formatting problems with multiple tables and echo: fenced #2769
• YAML error messages in the presence of parse errors are poor #385
• Add error message when {{< include >}} is used in invalid position #4291
• Warn Users When column and fig-cap-location interfere with one another (e.g. screen width figure with margin caption) #4150
• Document unsupported knitr chunk options #3371
• Lint: warn when knitr classes are used (was: use plot as blog post image) #8804
• Issue better errors/warnings in malformed labels #8386
• Add an error about label containing underscore  #7307
• quarto book .content-visible ignored #2140

See Mine's framing here: messages as a call to attention, warnings as a call to action.