Update lib/resty/core/socket.lua

Co-authored-by: lijunlong <lijunlong@openresty.com>

Author: chronolaw

lib/resty/core/socket.lua

@@ -165,7 +165,7 @@ local function setclientcert(cosocket, cert, pkey)
     end
 
     if type(cert) ~= "cdata" then
-        return nil, "bad client cert type"
+        return nil, "bad cert arg: cdata expected, got " .. type(cert)
     end
 
     if type(pkey) ~= "cdata" then