better error when 'w' is missing for windowed

``` R
file <- system.file('extdata', 'test_data.txt', package = 'sensorQC') 
sensor <- read(file, format="wide_burst", date.format="%m/%d/%Y %H:%M")
flag(sensor, 'x == 999999', 'persist(x) > 3', 'MAD(x,w) > 3', 'MAD(x) > 3')
 Error: object 'w' not found 
```

vs

``` R
sensor = window(sensor, type='auto')
flag(sensor, 'x == 999999', 'persist(x) > 3', 'MAD(x,w) > 3', 'MAD(x) > 3')
```

is fine
