renumbers error testers basing on 5000.

fixed a compiler bug not checking the lower bound of block depth in find_variable_backward_with_word()

t/var-5002.err

@@ -0,0 +1,3 @@
+## if you want local temporaries variables at the top-level, use the blocked expression.
+| a | ##ERROR: syntax error - variable declaration disallowed
+set a 10;