Fix error reporting and Theme Test Drive activation

There's an error when installing Theme Test Drive:
```
<br />
<b>Warning</b>:  Illegal offset type in isset or empty in <b>wp-includes/class-wp-error.php</b> on line <b>117</b><br />
ERROR: Failed to activate plugin: 
```
This is caused by passing the WP_Error object itself as the error code:
http://plugins.trac.wordpress.org/browser/developer/tags/1.1.2/developer.php#L364

Once I replaced that with `$activate_result->get_error_message()`, I got the proper message:
```
ERROR: Failed to activate plugin: Plugin file does not exist.
```
The plugin looks for `theme-test-drive/theme-test-drive.php` file, which doesn't exist. The correct file is `theme-test-drive/themedrive.php`.

SergeyBiryukov/developer@750025709f85532998d9f3bb96dc820df4623c11 fixes both of these issues.