fixes to make pushbuild tests green

mysql-test/t/plugin_load-master.opt:
  add '--loose' modifier to allow test to be skipped if plugin is not available.
sql/sql_plugin.cc:
  Failure to load a plugin from command line option made non-fatal as plugin
  load failures from plugin table is already non-fatal.

mysql-test/t/plugin_load-master.opt

 $EXAMPLE_PLUGIN_OPT
 "--plugin-load=;EXAMPLE=ha_example.so;"
---plugin-example-enum-var=e2
+--loose-plugin-example-enum-var=e2

sql/sql_plugin.cc

@@ -1181,9 +1181,8 @@ int plugin_init(int *argc, char **argv, int flags)
   /* Register all dynamic plugins */
   if (!(flags & PLUGIN_INIT_SKIP_DYNAMIC_LOADING))
   {
-    if (opt_plugin_load &&
-        plugin_load_list(&tmp_root, argc, argv, opt_plugin_load))
-      goto err;
+    if (opt_plugin_load)
+      plugin_load_list(&tmp_root, argc, argv, opt_plugin_load);
     if (!(flags & PLUGIN_INIT_SKIP_PLUGIN_TABLE))
       plugin_load(&tmp_root, argc, argv);
   }