Make validation exception more informative.

[swirtSJW]

As a person that has to debug validation failures, it would be helpful if the exception provided more details about what validation failed

Currently any failed validation says

RootedData\Exception\ValidationException: JSON Schema validation failed.

I am suggesting we iterate through the errors and list them so the output of the exception would be

RootedData\Exception\ValidationException: JSON Schema validation failed.
 1) data -> describedByType: application/pdf

Reproduction steps

Reproducing this is challenging as you need to start with invalid schema which is hard to save, because we don't allow invalid data.

1. First you need a dataset json that is invalid. (an empty downloadUrl or a describedByType = 7)
2. Then you can run it through $this->validMetadataFactory->get($json_string, $schema_id)
3. This should result in a validation exception being thrown. Without the PR it will just tell you it failed validation, with the PR it will tell you what element(s) and values filed validation.