More specific error messages for validation errors

[mcandre]

Describe the bug

When the user omits a required environment variable like NG_DEPLOY_AWS_REGION, then ng deploy reports a generic error message with no indication that the specific parameter is invalid.

Some environment variables like NG_DEPLOY_AWS_ACCESS_KEY_ID trigger messages about authentication problems. But the exact unbound environment variable name continues to be hidden from the poor user, who may not know what action to take to resolve the error.

To Reproduce
Steps to reproduce the behavior:

1. ng deploy

Expected behavior

Either a full list of invalid environment variables, or at minimum the first environment variable to fail validation, is shown to the user.

Desktop (please complete the following information):

• OS: macOS Catalina