patch 9.0.1604: errors from the codestyle test are a bit confusing

Problem:    Errors from the codestyle test are a bit confusing.
Solution:   Use assert_report() with a clearer message.  Avoid a warning for
            an existing swap file.

src/testdir/runtest.vim

@@ -111,9 +111,13 @@ if has("win32")
 else
   let s:test_script_fname = expand('%')
 endif
+
 au! SwapExists * call HandleSwapExists()
 func HandleSwapExists()
   if exists('g:ignoreSwapExists')
+    if type(g:ignoreSwapExists) == v:t_string
+      let v:swapchoice = g:ignoreSwapExists
+    endif
     return
   endif
   " Ignore finding a swap file for the test script (the user might be