Handle IR insertion errors gracefully

[dergoegge]

When adding new generators or changing the IR in some way we've seen bugs that break the assumptions certain generators make.

All the generators have code similar to:

builder
    .append(Instruction {
        inputs: vec![some_var.index, some_other_var.index],
        operation: Operation::XYZ,
    })
    .expect("Inserting XYZ should always succeed");

That is, they assume that they'll be able to insert some instruction into the program based on their assumption and if they can't we panic.

We should extend GeneratorError with e.g. a FailedInsertion type, such that we can catch and handle these errors gracefully. Additionally the error could carry useful information to debug the issue, as it very likely indicates a bug in the fuzzer.