hard panic when no #[fallback] is found

If you're missing the `#[fallback]` variant, you will see an error like
this:

```
error: proc-macro derive panicked
  --> crates/oxide/src/extractor/string_machine.rs:71:45
   |
71 | #[derive(Debug, Clone, Copy, PartialEq, Eq, ClassifyBytes)]
   |                                             ^^^^^^^^^^^^^
   |
   = help: message: A variant marked with #[fallback] is missing
```

crates/classification-macros/src/lib.rs

@@ -1,5 +1,4 @@
 use proc_macro::TokenStream;
-use proc_macro2::Span;
 use quote::quote;
 use syn::{
     parse_macro_input, punctuated::Punctuated, token::Comma, Attribute, Data, DataEnum,
@@ -84,7 +83,7 @@ pub fn classify_bytes_derive(input: TokenStream) -> TokenStream {
     }
 
     // If no fallback variant is found, default to "Other"
-    let fallback_ident = fallback_variant.unwrap_or_else(|| Ident::new("Other", Span::call_site()));
+    let fallback_ident = fallback_variant.expect("A variant marked with #[fallback] is missing");
 
     // For each of the 256 byte values, fill the table
     let fill = byte_map