DSLX: Confusing error message when spawning unknown proc

```
proc foo {
config {
spawn does_not_exist(...)(...);
()
}
next(tok: Token) { ... }
}
```

Trying to interpret this results in the error message "ParseError: Cannot find a definition for name: 'does_not_exist'", instead of "unknown proc, etc". Plz make user-friendlier.