fix(core): improve context providers error message for env-agnostic stacks

[eladb]

Following up on #2922: improve the error message emitted when trying to read context from an environment-agnostic stack by calling out CDK_DEFAULT_ACCOUNT and CDK_DEFAULT_REGION as a way to inherit environment information from the CLI.

Fixes #3078
Fixes #3120
Fixes #3130

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