There are several scenarios where commands fail because some of their dependencies are not met. The error that results in this can appear in several spots:

• when the actual command is run
• when another command is run that depends on that command
• when WP-CLI is run

While the two first error cases are not very elegant when working with WP-CLI, the third one is a hurdle a lot of people don't seem to be able to overcome. When something is wrong with their DB, they might not even be able to display the help screen.

To provide a generalized solution to overcome this, I'd like to look into providing a simple and flexible dependency resolution mechanism, with the ability to let commands that are currently installed remain inactive until all its dependencies are met.

Here's a mockup of what the user would see (in this example, with an invalid DB configuration):

This would keep WP-CLI working even when some dependencies are not met, and it would give clear indications to the user about what is currently missing for the command they need.