assert: add default operator to `assert.fail()`

This makes sure assert.fail() contains an operator instead of being undefined.

On top of that it also fixes the err.generatedMessage property. Before, it was not always set correct.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines