Adds error message to AliasExists exception class
v0.1.7

- Custom exception class definitions
    - They are mostly self-explanatory, but here is the break down:
        - AliasExists: raised when trying to create an alias that already exists in the database
        - DbInitError: raised when init_db has not been called
        - MailboxExists: raised when adding a mailbox that already exists in the database
        - NoSuchMailbox: raised when trying to access a mailbox that does not exist in the database
        - DomainExists: raised when trying to create a domain that already exists
        - NoSuchDomain: raised when trying to access a domain that does not exist in the database