test: :highlight validation errors

add test when highlight group doesn't exist.
add test when an invalid color name is used for `cterm`.

test/functional/ex_cmds/highlight_spec.lua

@@ -0,0 +1,43 @@
+local Screen = require('test.functional.ui.screen')
+local helpers = require("test.functional.helpers")(after_each)
+local eq, command = helpers.eq, helpers.command
+local clear = helpers.clear
+local eval, exc_exec = helpers.eval, helpers.exc_exec
+
+describe(':highlight', function()
+  local screen
+
+  before_each(function()
+    clear()
+    screen = Screen.new()
+    screen:attach()
+  end)
+
+  after_each(function()
+    screen:detach()
+  end)
+
+  it('invalid color name', function()
+    eq('Vim(highlight):E421: Color name or number not recognized: ctermfg=#181818',
+       exc_exec("highlight normal ctermfg=#181818"))
+    eq('Vim(highlight):E421: Color name or number not recognized: ctermbg=#181818',
+       exc_exec("highlight normal ctermbg=#181818"))
+  end)
+
+  it('invalid group name', function()
+    eq('Vim(highlight):E411: highlight group not found: foo',
+       exc_exec("highlight foo"))
+  end)
+
+  it('"Normal" foreground with red', function()
+    eq('', eval('synIDattr(hlID("Normal"), "fg", "cterm")'))
+    command('highlight normal ctermfg=red')
+    eq('9', eval('synIDattr(hlID("Normal"), "fg", "cterm")'))
+  end)
+
+  it('"Normal" background with red', function()
+    eq('', eval('synIDattr(hlID("Normal"), "bg", "cterm")'))
+    command('highlight normal ctermbg=red')
+    eq('9', eval('synIDattr(hlID("Normal"), "bg", "cterm")'))
+  end)
+end)