Clarify Valid Unit Types if Unit Type is Unrecognized

I'm trying to use prophet `cross_validation` but I received a value error for unrecognized type when I try:

```
df_cv = cross_validation(m, initial='120 months', period='12 months', horizon = '12 months')
```

I think the error should return a list of valid types so that I don't have to go look up why `months` aren't recognized as a valid unit.