assert: improve assertion errors

Please have a look at the commit messages for details.

This makes error messages in case of an error are improved while using assert.throws() and assert.rejects(). It also improves the error message for simple assertions when used in combination with new Function and adds a deprecation warning for validation functions that return anything besides true.

This should IMO be semver-minor, since it'll only impact code that fails the assertion (e.g., while writing new code). No passing assertion call is impacted by this.

@nodejs/assert PTAL

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