patch 8.2.5022: 'completefunc'/'omnifunc' error does not end completion

Problem:    'completefunc'/'omnifunc' error does not end completion.
Solution:   Check if there was an error or exception. (closes #10486,
            closes #4218)

src/testdir/test_ins_complete.vim

@@ -140,6 +140,30 @@ func Test_omni_dash()
   set omnifunc=
 endfunc
 
+func Test_omni_throw()
+  let g:CallCount = 0
+  func Omni(findstart, base)
+    let g:CallCount += 1
+    if a:findstart
+      throw "he he he"
+    endif
+  endfunc
+  set omnifunc=Omni
+  new
+  try
+    exe "normal ifoo\<C-x>\<C-o>"
+    call assert_false(v:true, 'command should have failed')
+  catch
+    call assert_exception('he he he')
+    call assert_equal(1, g:CallCount)
+  endtry
+
+  bwipe!
+  delfunc Omni
+  unlet g:CallCount
+  set omnifunc=
+endfunc
+
 func Test_omni_autoload()
   let save_rtp = &rtp
   set rtp=Xruntime/some