[DEX-90] Improve help and error messages for json-nd imports

[clemfromspace]

Small improvements for the commands expecting a json-nd input (synonyms, objects and rules commands):

• Adding a link to the json-nd specs in the help message:

  The file must contains one single JSON object per line (newline delimited JSON objects - ndjson format: https://ndjson.org/).

• Better error message when parsing issues are happening, with the line number:

  failed to parse JSON object on line 0: invalid character ',' after object key