Try to find missing configuration files or incorrect

In the forum are a lot of error descriptions to find,  which turns out it is a configuration problem. This commit handle missing files or incorrect paths.

```xml
<plugin name="poweradminbf4"/>
```
or
```xml
<plugin config="@bI_MAKE_A_MISSTAKEnf/plugin_poweradddminbf4.ini" name="poweradminbf4"/>
```
Will load `<full_path>\b3\\extplugins\\conf\\plugin_poweradminbf4.ini`


What do it?
- test if a given config file exists
- try to find a file if the given path is wrong or None

What is missing?
- tests