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_global.vim b/src/testdir/test_global.vim
index 38dc082..1e79ed1 100644
--- a/src/testdir/test_global.vim
+++ b/src/testdir/test_global.vim
@@ -36,7 +36,7 @@
 func Test_global_error()
   call assert_fails('g\\a', 'E10:')
   call assert_fails('g', 'E148:')
-  call assert_fails('g/\(/y', 'E476:')
+  call assert_fails('g/\(/y', 'E54:')
 endfunc
 
 " Test for printing lines using :g with different search patterns