thumbnailer panics when `THUMBNAILS_TXT_FONTMAP_FILE` does not exist

[rhafer]

Describe the bug

The thumbnailer panic when trying to render a preview of a text file and THUMBNAILS_TXT_FONTMAP_FILE points to a non existing file.

Error checks are missing here:
https://github.com/opencloud-eu/opencloud/blob/main/services/thumbnails/pkg/preprocessor/preprocessor.go#L145