metadata: fix incorrect tests

would always return false

similar to a50a24b0f408957885ff928569a061df07e5e993
Revert "metadata: properly handle media search type if we already have an extended inetref."

This reverts commit 30b03cc2b86e949b434b7afc207f12d35e6c6d87.
metadata: clear error state once an error has been propagated

Otherwise following successful downloads would have been marked as failed too
remotefile: Use more accurate MythCoreContext::IsThisHost(host) API

same as 654a4b203bebeeddfca088d0ca3f2a92c210ce89
metadata: add getStorageGroupName

remove duplicated code
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
metadata: properly update artwork images paths
metadata: create objects on stack

Data will actually be on heap, but so much more elegant
metadata: do not attempt to download remaining artworks should error occurred.

The downloaded data would have been discarded anyway
metadata: create Set/GetIsCollection

Will be used to propagate the information that the metadata came from a collection rather than a given season/episode.
That information is only necessary during the lifetime of the Metadatalookup object, so no need to change other classes
metadata: use extended inetref if available to determine the type of metadata
metadata: never overrides current video season/episode with with retrieved metadata ones

Only do so, if season or episode is 0
frontend: simplify metadata search creation

Use GuessLookupType API, no point duplicating the code all over the place
frontend: fix automatic mode introduced in previous commit
metadata: complete refactor of search

Use different sequence of searches in order to find the best matching possibility.
For TV, should no match be found, search for a generic artwork. This generic artwork will be automatically upgraded should specialised artwork ever be found.
This is done by setting a season number 0 for that generic artwork.
Original season/episode number will never get rewritten like it used it (was set to 1x1)