Add(InvalidStringFormatException).

Refactor(RegistrationDataValidatorTest): rename to RegexStringValidatorTest.
Modify(RegexStringValidator): throw InvalidStringFormatException on failed validation instead of returning a boolean.

Modify(UserRegistrationService): validate username and password before registering the user.
Refactor(RegexStringValidator): add parameter 'validatorName' to constructor.
Modify(InvalidStringFormatException): change constructor to message-constructor.

Modify(UserRegistrationPage): remove all class contents.
Modify(RegexStringValidatorTest): add test case for exception message to be same as validator name.
Modify(backend/pom.xml): add hibernate-validator dependency.