Fail immediately when there are wrong inputs for calcWaterYear

Fail helpfully when the wrong inputs are given.

```r
mydatedf <- data.frame("2015-10-20", stringsAsFactors = FALSE)
calcWaterYear(mydatedf)
Error in as.POSIXlt.default(dateVec) : 
  do not know how to convert 'dateVec' to class “POSIXlt”
Called from: as.POSIXlt.default(dateVec)
```