http.lua: Error informatively if insufficient LuaSec support

src/http.lua

@@ -35,7 +35,11 @@ local SCHEMES = {
     , https = {
         port = 443
         , create = function(t)
-            return require("ssl.https").tcp(t) end }}
+          local https = assert(
+            require("ssl.https"), 'LuaSocket: LuaSec not found')
+          local tcp = assert(
+            https.tcp, 'LuaSocket: Function tcp() not available from LuaSec')
+          return tcp(t) end }}
 
 -- default scheme and port for document retrieval
 local SCHEME = 'http'