best place to enfore rules

I'm new both to databases and postgres, so forgive me if this is a stupid
question.

Where do people usually enforce business rules? In the client application
or in the database?

For example, I might have a rule "don't allow customers to enter an order if
their account

is delinquent." I could create rules, triggers, etc. to prevent an entry
into the "order" table

given some condition in the "account" table. Or I could put the logic on
the client side.

I would think it would be better to do this inside the database. I'm not
familiar with how

the client would know what is happening. I guess the client can tell if an
SQL command

failed, but will the client know why it failed?