Print out which file is being validated or fails validation

[Remi-Gau]

I have to run the validation of an increasingly large number of JSONLD files and when one of them fails it becomes very hard to track down because the error message of validate are not the most informative.

Suggested fix:

• Option 1: make the error message more informative and report which file failed
• Option 2: (more verbose) prints which is validated

[Remi-Gau]

I think that this may also be solved by #40 as the lack of informative error message seesm to only happen when the file is not a valid json file.