There's one rather common failure mode with gpg, in that untrusted
keys are not accepted by gpgme for encryption.

A user can override that with --trust-model config or cmd line
params, but we can't do much from the gpgme client side:
https://www.gnupg.org/documentation/manuals/gnupg/GPG-Configuration-Options.html

Change-Id: Ia140a7fd25bd3f428aa11a7ceb0b7bdc47b2c900
Reviewed-on: https://gerrit.libreoffice.org/46060Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>