predict_metab omits MLE errors

[aappling-usgs]

For an MLE model where some days have been excluded (e.g., for NAs in DO.obs)

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,

get_fit(mm)

for that same model gives the appropriate errors in the errors column.