MB-6895 Better map function error isolation

If an error occurs for a map function of a view group,
don't discard the results from all other map functions
of the view group.

Change-Id: I49dde86e6698b018dade35d7ca7257f12ba53b86
Reviewed-on: http://review.couchbase.org/24205
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>