Validation failure and error messages from scripts

[rob-metalinkage]

Validation failed with a python error:

AttributeError: 'NoneType' object has no attribute 'read' === End JSON Schema ===

we should always provide some context if an exception occurs in code, and of course validation processes should never raise internal exceptions - they are confusing if one expects validation errors.

[avillar]

Fixed by 101fa60acad26102f671cff495554f8b7993085d, 0a591486a6faf8917c0257efce0ed8f314050bb2 and a8b5332cc4ad3cc3956ec0fd9a431c41a98cdd62