Make settings fun again - including loud but intelligible yelling if something is missing

[florianm]

Feature

All functions using settings should fail with a meaningful error message.

Source ropensci/software-review#335 @maelle

The current error messages are not informative in the absence of credentials, 
i.e. I get Error in curl::curl_fetch_memory(url, handle = handle) : <url> malformed 
if I run project_list() now without credentials. 
If there's no system variables set and no credentials passed either, 
the functions should fail with an informative error message.

[florianm]

It's probably time to bite the ckanr_setup/ckanr_settings apple I've avoided so far:

• get_odkc_{test_}{url, un, pw}() to wrap Sys.getenv("ODKC_{TEST_}{URL, UN,PW}") as standard args for functions, e.g. project_list(url = get_odkc_url(), un = get_odkc_un(), ...)
• ruODK_setup(url="xxx", un="yyy", pw="zzz", test_url="...", test_un="...", test_pw="...") to wrap Sys.setenv(ODKC_{TEST_}{URL, UN, PW})
  That would be an opportunity to make ruODK yell loudly but clearly about missing credentials.