use safe vim.eval/vim.command if possible (better tracebacks)

test/goto.vim

@@ -45,6 +45,7 @@ describe 'goto_simple'
     end
 end
 
+
 describe 'goto_with_tabs'
     before
         set filetype=python
@@ -72,6 +73,7 @@ describe 'goto_with_tabs'
     end
 end
 
+
 describe 'goto_with_buffers'
     before
         set filetype=python