Error message on accessing non-public function is ghastly

Here goes:

```
F0408 16:31:30.674867 8114 statusor.cc:64] Attempting to fetch value instead of handling error INVALID_ARGUMENT: TypeInferenceError: xls/modules/fma_32.x:29:14-29:19 <> Attempted to refer to module member fn fma(a: F32, b: F32, c: F32) -> F32 {
...
} that is not public.
*** Check failure stack trace: ***
@ 0x55b5402bda5c absl::logging_internal::LogMessage::DieIfFatal()
@ 0x55b5402bca8e absl::logging_internal::LogMessage::SendToLog()
@ 0x55b5402bc59f absl::logging_internal::LogMessage::Flush()
@ 0x55b54037fe2e absl::logging_internal::LogMessage::~LogMessage()
@ 0x55b5402bf1c2 AbseilInternalLog::Hook()
@ 0x55b54028b3ea absl::internal_statusor::ThrowBadStatusOrAccess()
@ 0x55b53fe5c19c xls::dslx::(anonymous namespace)::DeduceInvocation()
@ 0x55b53fe4fe49 xls::dslx::(anonymous namespace)::DeduceVisitor::HandleInvocation()
@ 0x55b53ff12f5b xls::dslx::Invocation::Accept()
@ 0x55b53fe4432c xls::dslx::Deduce()
@ 0x55b53fe43f74 std::__u::__function::__policy_invoker<>::__call_impl<>()
@ 0x55b53fe419e2 xls::dslx::DeduceCtx::Deduce()
@ 0x55b53fe445c2 xls::dslx::DeduceAndResolve()
@ 0x55b53fe3c200 xls::dslx::CheckTopNodeInModuleInternal()
@ 0x55b53fe3a7e1 xls::dslx::CheckTopNodeInModule()
@ 0x55b53fe3e1a6 xls::dslx::CheckModuleMember()
@ 0x55b53fe3db57 xls::dslx::CheckModule()
@ 0x55b53fdf5e3c main
@ 0x7f7fb2b49bbd __libc_start_main
@ 0x55b53fdf5029 _start
*** SIGABRT received by PID 8114 (TID 8114) on cpu 41 from PID 8114; ***
F0408 16:31:30.674867 8114 statusor.cc:64] Attempting to fetch value instead of handling error INVALID_ARGUMENT: TypeInferenceError: xls/modules/fma_32.x:29:14-29:19 <> Attempted to refer to module member fn fma(a: F32, b: F32, c: F32) -> F32 {
let ab = mul_no_round(a, b);
let greater_exp = (ab.bexp) if ((ab.bexp) > (((c.bexp) as u9))) else (((c.bexp) as u9));
let greater_sign = (ab.sign) if ((ab.bexp) > (((c.bexp) as u9))) else (c.sign);
let wide_c
E0408 16:31:30.701244 8114 process_state.cc:694] RAW: Raising signal 6 with default behavior
F0408 16:31:30.702869 8115 opt_main.cc:88] Check failed: ::absl::OkStatus() == (xls::RealMain(positional_arguments[0])) (OK vs. INVALID_ARGUMENT: Expected keyword 'package': Expected token, but found EOF.)
```

This was when executing the `xls/modules:fma_32_ir` genrule, i.e., during IR conversion. Yuck.