validations: include more details in `err.message`

Modify ValidationError constructor to include the model name and
a human-readable representation of the validation errors (messages)
in the error message.

Before this change, the message was pointing the reader
to `err.details`.  Most frameworks (e.g. express, mocha) log only
`err.message` but not other error properties, thus the logs were
rather unhelpful.

Example of the new error message:

    The `User` instance is not valid. Details: `name` can't be blank.