Handles HTTP 404 returned for no matches

Unfortunately the API also (currently, and erroneously) yields 404 for
other reasons than "not found", so this change confirms by checking the
error message from the server.

Fixes #151.

R/pkb_get.R

@@ -36,9 +36,16 @@ get_json_data <- function(url, query,
   else
     res <- httr::POST(url, httr::accept_json(), httr::user_agent(ua()),
                       body = query, encode = "form")
+  # some endpoints return HTTP 404 for failure to find data, though some
+  # also (erroneously) do so for other reasons
+  contLen <- res$headers$`content-length`
+  if (res$status_code == 404 && (! is.null(contLen)) && contLen > 0) {
+    # confirm by checking the error message
+    err <- httr::content(res, as = "text")
+    if (endsWith(err, "resource could not be found.")) return(NULL)
+  }
   stop_for_pk_status(res)
   # some endpoints return zero content for failure to find data
-  contLen <- res$headers$`content-length`
   if ((! is.null(contLen)) && contLen == 0) return(NULL)
 
   # if content-type is application/json, httr:content() doesn't assume UTF-8