use ${project.parent.artifactId} instead of deprecated ${parent.artifactId}
add unique constraint to username column
throw UsernameAlreadyExistsException if JpaSystemException threw while new user persisting
add UsernameAlreadyExistsException
throw IllegalArgumentException while deleting user if id doesn't exist
refactor UsernameAlreadyExistsException
add UserIdNotFoundException
throw UserIdNotFoundException if user with passed id doesn't exist
add tests for RegistrationServiceImpl
move exceptions to exception package
add exception handling