[ExportVerilog][SV] Emit an error for invalid symbol substitutions

```mlir
sv.verbatim "{{0}}" {symbols = [@Foo]}
```

Currently `circt-opt -export-verilog` doesn't raise an error even though `@Foo` doesn't exist. Ideally we should have a verifier to reject such IR in the first place (I guess there is on going work regarding SymbolUserInterface?) but at least we should cause a hard error at ExportVerilog.

Start by reproducing the issue with the shown sv.verbatim IR and circt-opt -export-verilog. Trace the ExportVerilog handling of symbol substitutions and ensure an unresolved @Foo produces a hard error; the existing behavior should be covered by a regression test if the relevant test location is found.

Written by the indexing model from the issue text.