Promote download when preview is not available

This is in the 3011-recline-updates branch, needs to be merged into master for CKAN 2.0.

Users shouldn't see red error message just because the data preview isn't working. There should be a message along the lines of "Ne preview available for this data" or "for this data at this time" if it's s temp bug, instead of current messages and download / data API links should be more prominent