Body update fail with read as plain

* src/mail.cpp: Add an attachment wth the body or errormessage

Create an *.txt attachment with the content of the body or the
errormessage from the parser unless
that getting the Body after putting the placeholder into
the Body did return a non empty string.
Remove code to deselect the mail if the above check fails.

• GnuPG-Bug-Id: T7858