DSLX: Bad error message on malformed `match` expr

I wrote bad code like the following:
```
let new_step = match (state.step, aad_blocks_left == u32:0, ctxt_blocks_left == u32:0) {
( Step::RECV_AAD, false) => Step::RECV_AAD,
...
};
```

Notice that the number of fields in the matcher differs from that in the match header. I'd expect some "bad number of whatever" error message, but instead I got:

```
E0824 09:51:56.399187 2649910 run_routines.cc:332] Internal error: INTERNAL: Could not find type for number: false
```

Should be fixed.