### What
* Narrow down the metadata base warnings only when there's any relative
urls need to be resolved, if there's only absolute urls present, no need
to resolve and we don't warn.
* Polish the error message, updated from "metadata.metadataBase is not
set ..." to "metadataBase property in metadata export is not set ..."
### Why
It will be confusing if we're still show metadataBase warning when
there's no need to set one, since the social image cards only have
absolute urls
Closes NEXT-2426
3c4ec650b3