prevent exporttree=yes on remotes that don't support exports

Don't allow "exporttree=yes" to be set when the special remote
does not support exports. That would be confusing since the user would
set up a special remote for exports, but `git annex export` to it would
later fail.

This commit was supported by the NSF-funded DataLad project.

Remote.hs

@@ -53,6 +53,7 @@ module Remote (
 	checkAvailable,
 	isXMPPRemote,
 	claimingUrl,
+	isExportSupported,
 ) where
 
 import Data.Ord