EditorError set

If you try to compile now, the compiler will tell you that this error doesn't exist. If you try to remove it from the PromptCb return value, the compiler will tell you

error: function type cannot have an inferred error set

So we need an explicit error set for our callback. We don't know how many kinds of errors could cause a PromptCb to fail. The callback we'll be using for the searching function will be of type

error{OutOfMemory}

So we could just write that. But PromptCb is a 'generic' callback, which could do just about anything, and we'd need to add more errors to that set.

Instead, we create our EditorError set, and if we'll need to handle more errors, we'll add them to this set.

Just add it above our previous IoError set:

/// Error set for functions requiring explicit error handling.
pub const EditorError = error{
    OutOfMemory,
};