patch 8.2.1183: assert_fails() checks the last error message

Problem:    assert_fails() checks the last error message.
Solution:   Check the first error, it is more relevant.  Fix all the tests
            that rely on the old behavior.
diff --git a/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index 6d119fe..6005b8d 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -693,7 +693,7 @@
   " Search for non existing help string
   call assert_fails('Xhelpgrep a1b2c3', 'E480:')
   " Invalid regular expression
-  call assert_fails('Xhelpgrep \@<!', 'E480:')
+  call assert_fails('Xhelpgrep \@<!', 'E866:')
 endfunc
 
 func Test_helpgrep()
@@ -3748,8 +3748,8 @@
 
 func Test_lvimgrep_crash2()
   au BufNewFile x sfind
-  call assert_fails('lvimgrep x x', 'E480:')
-  call assert_fails('lvimgrep x x x', 'E480:')
+  call assert_fails('lvimgrep x x', 'E471:')
+  call assert_fails('lvimgrep x x x', 'E471:')
 
   au! BufNewFile
 endfunc