validations: include more details in `err.message`

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

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

Example of the new error message:

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