ohos: Deduplicate translated messages

Introduce helper functions into OhosMapKit.
As a drive-by, use errorString() consistently instead
of the enum value.

Pick-to: 6.12
Change-Id: Ib14cd0f73d530d4097943a427177e89a945d62a2
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Dawid Śliwa <dawid.sliwa@qt.io>