Error if no mapping files are present and oscf conversion is true

Currently if you set `DO_OCSF_CONVERSION` to true and no mapping files are provided the script will consume events like normal without doing conversions.

I'd prefer it to error when no mapping files are found.

Possible error message:

```
No mapping files found in './ocsf/mappings'. Please provide mapping files or set 'DO_OCSF_CONVERSION' to false.
```