Prevents erroneous creation of discrete events (vector of conditions)

[TorkelE]

Should close #2612

Looking at the code, my impression is that stuff like

@parameters p(t) d
@variables X(t)
SymbolicDiscreteCallback([X < 5.0, X > 10.0] => [X ~ Pre(X) + 10.0]

should not be allowed. It eventually yields an error (but a hard-to-interpret one). This throws an error as soon as someone tries to create the event (happy to modify if this is something we'd actually want to supprot).