Lua filters: improve error messages for failing filters

[tarleb]

Print the Lua error properly instead of displaying their show string.

[tarleb]

This makes error output better, but issues remain. E.g.:

-- test.lua
function Pandoc (doc)
  pandoc.mediabag.fetch('does-not-exist.txt')
end

This triggers a PandocResourceNotFound error, which is still just shown:

Error running filter ./test.lua:
PandocResourceNotFound "does-not-exist.txt"
stack traceback:
        [C]: in function 'pandoc.mediabag.fetch'
        ./test.lua:10: in function 'Pandoc'

One solution could be to add a new PandocError constructor PandocErrorStack (NonEmpty PandocError).

[jgm]

This is good -- the other case you mention doesn't seem bad to me.