fix handling of end labels

- disallow using end label alone on blocks
- improved parse error for mismatches
- add label checking for non-block constructs
- allow generate block to have label before begin

test/error/end_label_function_wrong.sv

@@ -0,0 +1,6 @@
+// pattern: element "yes" has mismatched end label "no"
+module top;
+    function yes;
+        yes = 0;
+    endfunction : no
+endmodule