predict_metab omits MLE errors

For an MLE model where some days have been excluded (e.g., for `NAs in DO.obs`)
```r
predict_metab(mm)
```
yields a data.frame where the `errors` column is NA whenever a day is excluded, rather than reporting on the reason for the exclusion. In contrast,
```r
get_fit(mm)
```
for that same model gives the appropriate errors in the `errors` column.

Start by reproducing the example with an MLE model containing excluded days, then inspect predict_metab(mm) and compare its errors handling with get_fit(mm). Done means predict_metab reports the exclusion reasons in its errors column instead of NA, consistently with get_fit.

Written by the indexing model from the issue text.