corecontext: QHostAddress::isNull() appears buggy

It doesn’t reliably returns true when provided an invalid IP address. So get around that
metadata: clear error state once an error has been propagated

Otherwise following successful downloads would have been marked as failed too

(cherry picked from commit 085c65e91273bbeb3e2d70298c795a027406a62b)
metadata: add getStorageGroupName

remove duplicated code

(cherry picked from commit 6954cff4005271b6360fdf25a4e2f7781a8e108a)
metadata: fix storage group access when downloading artworks.

Check for an existing file was all broken, would always download no matter what
Also, make sure only the master backend access the file system directly

(cherry picked from commit 1e844849c6bb6e94e2a4e661ed038848a18ed06d)

Conflicts:
	mythtv/libs/libmythmetadata/metadataimagedownload.cpp
metadata: properly update artwork images paths

(cherry picked from commit 823e8511070c122657ed22238a40ab41cec0c545)
metadata: create objects on stack

Data will actually be on heap, but so much more elegant

(cherry picked from commit 1f21438d15a01e66e034233f436a969bece97869)
metadata: do not attempt to download remaining artworks should error occurred.

The downloaded data would have been discarded anyway

(cherry picked from commit eef41b99470a8f94125e8ee0848ff0c0a5cc265a)
Bump ABI following a34d6e41fe419acb2f05853ce9fceb371b9bc84d