Show meaningful error message if `data`-folder is empty

[sroertgen]

If there is no data in the data folder there is currently some cryptic error messages, which do not point to the cause of error, but give something like that:

Encountered an error parsing the provided GraphQL type definitions:
Syntax Error: Expected Name, found "}".

  37 |     hub: String
  38 |   }
  39 |
  40 |   type LanguageMap {
  41 |
> 42 |   }
     |   ^
  43 |
  44 |   type LanguageMapArray {
  45 |
  46 |   }
  47 |