Improved error message when writting a wrong output format

[jspaezp]

I noticed that when rendering a document, and attempting to use md as the output format, it just fails.
It would be great if in addition it mentioned what strings are acceptable in the field.

$ quarto --version
1.1.179

$ quarto render source/file.qmd --to=md
ERROR: Invalid format string


$ quarto render source/file.qmd --to=gfm

Executing 'file.ipynb'
  Cell 1/1...Done

pandoc 
  to: gfm+footnotes+tex_math_dollars-yaml_metadata_block
  output-file: file.md
  standalone: true
  default-image-extension: png
  
Output created: file.md

$ quarto render source/file.qmd --to=markdown

Executing 'file.ipynb'
  Cell 1/1...Done

pandoc 
  to: markdown
  output-file: file.md
  standalone: true
  default-image-extension: png
  

In addition, this feels inconsistent, since running the render with a document of md extension works

$ quarto render source/file.qmd --output output/file.md

Starting python3 kernel...Done

Executing 'file.ipynb'
  Cell 1/1...Done

pandoc --output ../output/file.md
  to: html
  standalone: true
  section-divs: true
  html-math-method: mathjax
  wrap: none
  default-image-extension: png
  
metadata
  document-css: false
  link-citations: true
  date-format: long
  lang: en
  
Output created: ../output/file.md

[cscheid]

This is a pandoc feature which we don't control:

$ quarto pandoc --list-output-formats
asciidoc
asciidoctor
beamer
biblatex
bibtex
commonmark
commonmark_x
context
csljson
docbook
docbook4
docbook5
docx
dokuwiki
dzslides
epub
epub2
epub3
fb2
gfm
haddock
html
html4
html5
icml
ipynb
jats
jats_archiving
jats_articleauthoring
jats_publishing
jira
json
latex
man
markdown
markdown_github
markdown_mmd
markdown_phpextra
markdown_strict
markua
mediawiki
ms
muse
native
odt
opendocument
opml
org
pdf
plain
pptx
revealjs
rst
rtf
s5
slideous
slidy
tei
texinfo
textile
xwiki
zimwiki