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_method.vim b/src/testdir/test_method.vim
index 7c0559a..0d55f2a 100644
--- a/src/testdir/test_method.vim
+++ b/src/testdir/test_method.vim
@@ -131,9 +131,9 @@
   eval [1, 2, 3]  
 	\ ->sort(
 	\ )
-  call assert_fails('eval [1, 2, 3]-> sort()', 'E260:')
+  call assert_fails('eval [1, 2, 3]-> sort()', 'E15:')
   call assert_fails('eval [1, 2, 3]->sort ()', 'E274:')
-  call assert_fails('eval [1, 2, 3]-> sort ()', 'E260:')
+  call assert_fails('eval [1, 2, 3]-> sort ()', 'E15:')
 endfunc
 
 func Test_method_lambda()