Exporting matrices fails with uninformative error when dimnames contain invalid Stata names

Reproducible example:

```
rcall clear
rcall: A = matrix(1:6, nrow=2, byrow = TRUE, dimnames = list(c("A_1","B_2"),c("X.1","X.2","X.3")))
return list
mat list r(A)
```

The script fails with "factor-variable and time-series operators not allowed"