use std::optional for this error field

otherwise when I add source/line to the uno::Exception in an
upcoming change, the message becomes non-empty, and hasErrors()
return true.

I note that we throw SQLException in lots of places without a
message, so it is possible that this change will cause us to notice
errors we previously missed.

Change-Id: Idf7c5b5143e20c992d8d7550043aa2ed875c78b1
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/94108
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

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