buck2_validation: tweak and slightly improve error message

[thoughtpolice]

Previously, when using ValidationInfo() with a failure, it ends up turning out an awkward message that turns

{ "status": "failure", "message": "you failed the test!" }

into

"you failed the test!".

After this patch, it comes out as:

you failed the test!

Which seems much more reasonable and easy to parse. It also gives the validation message more control over the punctuation.