restore the error message that was changed by mistake

add a test case

mysql-test/main/plugin.result

 CREATE TABLE t1(a int) ENGINE=EXAMPLE;
 ERROR 42000: Unknown storage engine 'EXAMPLE'
+INSTALL PLUGIN non_exist SONAME 'ha_example';
+ERROR HY000: Can't find symbol 'non_exist' in library 'ha_example.so'
 INSTALL PLUGIN example SONAME 'ha_example';
 INSTALL PLUGIN EXAMPLE SONAME 'ha_example';
 ERROR HY000: Plugin 'EXAMPLE' already installed

mysql-test/main/plugin.test

@@ -4,6 +4,10 @@
 --error ER_UNKNOWN_STORAGE_ENGINE
 CREATE TABLE t1(a int) ENGINE=EXAMPLE;
 
+--replace_regex /\.dll/.so/
+--error ER_CANT_FIND_DL_ENTRY
+INSTALL PLUGIN non_exist SONAME 'ha_example';
+
 INSTALL PLUGIN example SONAME 'ha_example';
 --replace_regex /\.dll/.so/
 --error ER_PLUGIN_INSTALLED

sql/sql_plugin.cc

@@ -1222,7 +1222,8 @@ static enum install_status plugin_add(MEM_ROOT *tmp_root, bool if_not_exists,
   DBUG_ASSERT(!name->str || !dupes); // dupes is ONLY for name->str == 0
 
   if (errs == 0 && oks == 0 && !dupes) // no plugin was found
-    my_error(ER_PLUGIN_IS_NOT_LOADED, MyFlags, name->str);
+    my_error(ER_CANT_FIND_DL_ENTRY, MyFlags, name->str, tmp.plugin_dl->dl.str);
+
 
   plugin_dl_del(tmp.plugin_dl);
   if (errs > 0 || oks + dupes == 0)