Handle "swap file found" (E325) exception

The user has been warned and asked about how to proceed already.
Catching this exception removes the additional output afterwards:

  Error detected while processing function <SNR>237_open_buffers:
  line   38:
  E325: ATTENTION

Closes #67.

autoload/startify.vim

@@ -502,8 +502,9 @@ function! s:open_buffers(cword) abort
   else
     try
       execute 'normal' a:cword
-    catch /E832/
+    catch /E832/  " don't ask for undo encryption key twice
       edit
+    catch /E325/  " swap file found
     endtry
   endif
 endfunction