Sourcing lies about adding triples

[retog]

When adding to an inexistent dataset sourcing says:

"Added 220 triples to urn:x-localinstance:/dlc/marec/rdf.graph"

Should get a 400 code and an error message instead. Or even just letting the exception propagate leading to a 500 error would be better than the current behavior.

[retog]

@luigiselmi I think this should be fixed. In my cleaning actions I've certainly removed all code swallowing exceptions I came across.