Show validation error messages

[dcaribou]

Context

A FailedAssetValidation is raised whenever a validation error happens for an asset in during preparation. This exception, though, does not currently display the error message, which means that in order to understand what the problem is we need to look at the files with the validation report that are produced in transfermarkt_datasets/

Solution

The FailedAssetValidation exception could contain a error message that summarised the validation report, which would allow understanding what the error could be from the logs.