Better handling of connection string

[joehan]

Description

This PR does 2 things:

• Adding Postgres setup to 'setup:emulators:dataconnect'
• Error out if no connection string is set when we try to connect to postgres.

Scenarios Tested

Running 'firebase setup:emulators:dataconnect' from outside a project directory, then inside a project directory

Try to start emulator with a connection string configured, error out, run the suggested command, and then successfully run the emulator