core: diff should imply --fail by default

[l0b0]

Describe the bug

Rather than a separate --fail flag, cdk diff should return a non-zero exit code if there is a diff. This would provide more information to the end user, making it easier to automate cdk diff commands.

Expected Behavior

cdk diff should return zero if there is no difference, and a non-zero exit code if there is a difference.

Current Behavior

cdk diff returns a zero exit code if there is a difference.

Reproduction Steps

Run cdk diff on a stack with differences.

Possible Solution

No response

Additional Information/Context

No response

CDK CLI Version

2.93.0 (build 724bd01)

Framework Version

No response

Node.js Version

v18.17.1

OS

NixOS

Language

Typescript

Language Version

5.2.2

Other information

No response