I think it'd be nice to have a whole set of possible messages, each associated with some equation which has zero on the right hand side. It'd look something like this:

"404; Did you just [insert operation that yields zero*]? Because there's nothing here! (Insert relevant equation**)"

*Examples; (**Corresponding equations):

1. symmetrize the electromagnetic field strength tensor \(\bigl(F^{(\mu\nu)}=0\bigr)\)

2. take a covariant derivative of the metric \(\bigl(\nabla_\rho g_{\mu\nu}=0\bigr)\)

3. calculate a lightlike interval \(\bigl(ds^2=0\bigr)\)

4. vary the action of the internet \(\bigl(\delta S=0\bigr)\)

5. take the d'Alembertian of a massless field \(\bigl(\square\phi=0\bigr)\)

6. check the Bianchi identities \(\bigl(\nabla_{[\mu}F_{\nu\sigma]}=0\bigr)\ \text{or}\ \bigl(\nabla_{[\mu}R_{\nu\sigma]}=0\bigr)\)

feel free to add other/better ones if you'd like