When inserting of email bodypart failes, insert a failure message
to the buffer (and continue) instead of halting the insertion of
the rest of that email thread in question.
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This is (now real !!!) alternative to

id:"1350824336-11060-1-git-send-email-tomi.ollila@iki.fi"

This adds error message string to the output, error message in
id:"87d31artti.fsf@inf-8657.int-evry.fr" is shown as

  !!! Bodypart insert error: Wrong type argument: stringp, nil !!!