DSLX: Confounding error message on not-found imported function

When writing the AES-GCM code, I mistakenly referenced `aes_128::aes_128_encrypt`, which doesn't exist, instead of `aes_128::aes_encrypt`, which does. Rather than getting a nice "Could not find function"-style error, I instead got:

```
F0823 12:36:06.904267 2167948 interpreter_main.cc:152] Check failed: ::absl::OkStatus() == (status) (OK vs. NOT_FOUND: No N3xls4dslx8FunctionE in module google3.third_party.xls.modules.aes.aes_128 with name "aes_128_encrypt"
```

WTH is up with that...mangled-looking "N3xls4dslx8FunctionE"?