Sorry, the previous patch was incomplete. The attached patch is correct.

Best regards,

Juan Manuel 

Juan Manuel Macías writes:

> Rationale for the attached patch: It seems that a common problem that
> users have with exporting to LaTeX is unicode characters that cannot be
> represented in pdfLaTeX or LuaLaTeX/XelaTeX. In the Unicode TeX engines
> the warning is insidious, since the missing character warning is not
> preceded by a 'warning' string.
>
> Naturally, the added Org warnings do not solve the problem, but at least
> they give some clues on how to properly adjust the document.