ex_getln: Fix “echoerr msg not shown” problem

This also attempted to fix problem with cancelling input() on error by avoiding 
standard error printing facilities (assumed thrown error message is the 
problem), but with no luck so far.

test/functional/ui/cmdline_highlight_spec.lua

@@ -289,7 +289,16 @@ describe('Command-line coloring', function()
   it('does the right thing when errorring', function()
     set_color_cb('Echoerring')
     start_prompt('e')
-    -- FIXME Does not work well with :echoerr: error message not shown.
+    screen:expect([[
+      {EOB:~                                       }|
+      {EOB:~                                       }|
+      {EOB:~                                       }|
+      {EOB:~                                       }|
+      :                                       |
+      {ERR:E5407: Callback has thrown an exception:}|
+      {ERR: Vim(echoerr):HERE}                      |
+      :e^                                      |
+    ]])
   end)
   it('does the right thing when throwing', function()
     set_color_cb('Throwing')