check `file_path` before dowload begins in `getRaster`

[geryan]

feature request

Hi MAP team!

If file_path is somehow misspecified,getRaster only fails at the end of a long slow download. Annoying for me, I suspect VV frustrating for colleagues with less good internet.

Something like stopifnot(dir.exists(file_path) at the beginning of the function should solve this.

Related: the writeRaster(...... overwrite = FALSE) hard coding would also lead to (unnecessary) failure after a long download if the file already existed. Checking if the target file exists before downloading would be kind - this is how geodata::gadm operates, i.e., if files exist it just reads them in instead of re-downloading.

[mauricio-tki]

Hi Gerry, thanks for filing this. Checking the path beforehand is definitely better practice, I'll look into improving this soon.