better ExtractAndCombData warning message

[goldingn]

This pops up a lot:

Warning message:
In ExtractAndCombData(x, covariate.output[[1]]) :
  38 extracted covariate values are NA. This may cause issues for some models

but it isn't immediately obvious that it refers to occurrences falling where there are missing covariate values. Could do with rewording.

[AugustT]

Yeah, it makes sense to me but maybe not to a new user. Maybe change to something like "X occurrence points fall outside your covariate raster and therefore have NA values for covariates..."

[goldingn]

yeah, that's perfect