fix a minor issue when echoing the current error

Only clear the cmd window when it is displaying a message that we
echoed. Previously we were echoing a blank line every time the cursor
moved - this was clearing messages generated from other sources than
syntastic.

plugin/syntastic.vim

@@ -340,12 +340,16 @@ function! s:EchoCurrentError()
     let lnum = line(".")
     for i in b:syntastic_loclist
         if lnum == i['lnum']
+            let b:syntastic_echoing_error = 1
             return s:WideMsg(i['text'])
         endif
     endfor
 
     "Otherwise, clear the status line
+    if exists("b:syntastic_echoing_error")
         echo
+        unlet b:syntastic_echoing_error
+    endif
 endfunction
 
 "return a string representing the state of buffer according to