loading replay of unsupported map throws an error

[MildlyInterested]

1. Save replay e.g. on cfb_moosehead map (unsupported at this date)
2. Select replay on website.
3. Observe error

Suggested fix: More verbose error explaining the map can't be loaded.

I also remember there being a mention of falling back to a cloud background if no map can be found but can't find that statement atm.