MM: detect save error based on the return value

There is actually GetError() and GetErrorCode(). While the error
code can also be a warning, which isn't reported by GetError(),
this detects the error based on the DoSaveAs return value (actually
I got warnings in case of save failures in my MM unit tests).

Change-Id: I3db749c798ad80336ee7ada9eaa71177e8ffb248
Reviewed-on: https://gerrit.libreoffice.org/11456
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>