Fix cryptic blueprint validation error messages

[reimic]

When a blueprint fails validation a cryptic error message is returned.

For this failing blueprint (which should not fail, but that is addressed in #83):

$blueprint = BlueprintBuilder::create()
    ->withWordPressVersion( 'https://wordpress.org/latest.zip' )
    ->toBlueprint();
...results in this error:

...this is the error message provided:

Array
(
    [/constants] => Array
        (
            [0] => The data (array) must match the type: object
        )

)

We can, and in fact - we should do better.

[adamziel]

Duplicate of #83