Now there is no capability to skip failed assertion during test execution, TestCafe considers that something goes wrong if assertion doesn't pass and the test fails.
If you want to write assertion which doesn't influence on test passing, you can write e.g. the following code:
// If some element is visible I want check that its text equal to 'some text'
if (await page.msgErreur.visible)
await t.expect(page.msgErreur.value).eql('some text');
We think that it is better to write straight and predictable logic in a test.