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.

Best regards,

Juan Manuel 

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Juan Manuel Macías -- Composición tipográfica, tratamiento de datos, diseño editorial y ortotipografía