Clarify error message (#931)

The existing error message printed when plug#end() is called without
calling plug#begin() doesn't make the dependence on plug#begin()
obvious; I had to go digging in the vim-plug code to discover what I'd
done wrong. This attempts to clarify the error a bit, to make it more
obvious to a user.

plug.vim

@@ -251,7 +251,7 @@ endfunction
 
 function! plug#end()
   if !exists('g:plugs')
-    return s:err('Call plug#begin() first')
+    return s:err('plug#end() called without calling plug#begin() first')
   endif
 
   if exists('#PlugLOD')