Fix defect #1693: Unhandled IntegrityError on /admin/add
Fix defect #1693: Unhandled IntegrityError on /admin/add

This fixes an unhandled IntegrityError produced when an admin
tried to make an account with the same username as an existing
user. Caught during QA

micahflee/securedrop,heartsucker/securedrop,conorsch/securedrop,ageis/securedrop,ageis/securedrop,ageis/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,conorsch/securedrop,garrettr/securedrop,ehartsuyker/securedrop,garrettr/securedrop,conorsch/securedrop,ageis/securedrop,micahflee/securedrop,micahflee/securedrop,conorsch/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,micahflee/securedrop,heartsucker/securedrop