Check if names supporting checking terminals exist.

[dblodgett-usgs]

I now see:

> library(nhdplusTools)

> temp_dir = file.path(nhdplusTools_data_dir(), "temp_hr_cache")

> download_dir = download_nhdplushr(temp_dir, c("1403", "1405", "1406", "1401", "1402"))
  |=====================================================================================================| 100%

> nhdflowline = get_nhdplushr(download_dir, file.path(download_dir, "nhdplus_out.gpkg"),
+                             layers="NHDFlowline",
+         .... [TRUNCATED] 
Warning message:
In get_nhdplushr(download_dir, file.path(download_dir, "nhdplus_out.gpkg"),  :
  check_terminals is true but attributes selected do not support the checks.