Add link-to-search on "Package not found" page

Fixes #440

Author: Soorya Narayan

Distribution/Server/Features/PreferredVersions.hs

@@ -264,7 +264,7 @@ versionsFeature ServerEnv{ serverVerbosity = verbosity }
     withPackagePreferred pkgid func = do
       pkgIndex <- queryGetPackageIndex
       case PackageIndex.lookupPackageName pkgIndex (packageName pkgid) of
-            []   ->  packageError [MText "No such package in package index"]
+            []   ->  packageError [MText $ "No such package in package index. ", MLink "Search for related terms instead?" $ "/packages/search?terms=" ++ (display $ pkgName pkgid)]
             pkgs  | pkgVersion pkgid == nullVersion -> queryState preferredState (GetPreferredInfo $ packageName pkgid) >>= \info -> do
                 let rangeToCheck = sumRange info
                 case maybe id (\r -> filter (flip withinRange r . packageVersion)) rangeToCheck pkgs of