Aid the user in setting up filterchains by better logmessages

The builtin filterchain writer module requires to know the name of the writer module to send the data to. I didn't find any documentation about how the writers names were created; some of them are dynamicaly generated depending on the configuration. 
This patch outputs the name of all configured writers right next to the errormessage of being unable to choose one so the user can succeed in generating his config without source reading and documentation nightmares.
this is unnecessary, just allocate for the subsequent list you're going to build, like so:

```c
strjoin(str, str, keys, n, "', '");
INFO("%s ['%s']", comment, str);
```