patch 8.0.0511: message for skipping client-server tests is unclear

Problem:    Menuage for skipping client-server tests is unclear.
Solution:   Be more specific about what's missing (Hirohito Higashi, Kazunobu
            Kuriyama)

src/testdir/test_clientserver.vim

@@ -11,13 +11,15 @@ func Test_client_server()
   if cmd == ''
     return
   endif
-  if has('unix')
+  if has('x11')
+    if empty($DISPLAY)
+      throw 'Skipped: $DISPLAY is not set'
+    endif
     try
       call remote_send('xxx', '')
     catch
       if v:exception =~ 'E240:'
-	" No connection to the X server, give up.
+	throw 'Skipped: no connection to the X server'
-	return
       endif
       " ignore other errors
     endtry

src/testdir/test_quotestar.vim

@@ -118,8 +118,12 @@ func Test_quotestar()
 
   if has('macunix')
     let skipped = Do_test_quotestar_for_macunix()
-  elseif !empty("$DISPLAY")
+  elseif has('x11')
+    if empty($DISPLAY)
+      let skipped = "Test can only run when $DISPLAY is set."
+    else
       let skipped = Do_test_quotestar_for_x11()
+    endif
   else
     let skipped = "Test is not implemented yet for this platform."
   endif

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    511,
 /**/
     510,
 /**/