Show appropriate error message for model validation error

Before:
Possibly unhandled SequelizeValidationError: Validation error
    at /home/phanect/dev/notel-api/node_modules/sequelize/lib/instance-validator.js:150:14
    ...

After:
Possibly unhandled SequelizeValidationError: field_name cannot be null
    at /home/phanect/dev/notel-api/node_modules/sequelize/lib/instance-validator.js:150:14
    ...