Improve error message when internet off or endpoint unreachable

[frankfarach]

From ropensci/software-review#505 (comment):

Because this is a recurrent theme for API packages (and CRAN asks them to "fail gracefully" in these cases), I've checked what happens when I turn of the internet and I don't think it is that explicit for the user.

> npi_search(city = "San Francisco")
Requesting records 0-10...
Error in curl::curl_fetch_memory(url, handle = handle) : 
  Could not resolve host: npiregistry.cms.hhs.gov

Maybe there could be a way to display a better error message when internet is off or the website unreachable?
Like check first hand if internet is off and errors explicitly if it is the case.

[frankfarach]

I was able to reproduce the reported issue and fix it. Here is a reprex after fixing it:

npi::npi_search(city = "San Francisco")
#> 10 records requested
#> Requesting records 0-10...
#> Error in `npi_api()` at npi/R/api.R:49:2:
#> ! Unable to access Internet. Check network connection and try again.