should throw if modifier is not valid

[blond]

Actual

new BemEntityName({
    block: 'button',
    mod: { val: 'action' }
});

// ➜ BemEntityName { block: 'button' }

new BemEntityName({
    block: 'button',
    mod: {}
});

// ➜ BemEntityName { block: 'button' }

Expected

new BemEntityName({
    block: 'button',
    mod: { val: 'action' }
});

// ➜ This is not valid BEM entity: the field `mod.name` is undefined.

new BemEntityName({
    block: 'button',
    mod: {}
});

// ➜ This is not valid BEM entity: the field `mod.name` is undefined.