patch 8.1.0973: pattern with syntax error gives threee error messages

Problem:    Pattern with syntax error gives threee error messages. (Kuang-che
            Wu)
Solution:   Remove outdated internal error.  Don't fall back to other engine
            after an error.
diff --git a/src/testdir/test_search.vim b/src/testdir/test_search.vim
index 4ff2aca..5cf70d4 100644
--- a/src/testdir/test_search.vim
+++ b/src/testdir/test_search.vim
@@ -1220,3 +1220,8 @@
     call assert_match('E678:', v:exception)
   endtry
 endfunc
+
+func Test_one_error_msg()
+  " This  was also giving an internal error
+  call assert_fails('call search(" \\((\\v[[=P=]]){185}+             ")', 'E871:')
+endfunc