Combine withCallingHandlers with tryCatch to catch keyring errors

see:
https://cran.r-project.org/web/packages/tryCatchLog/vignettes/tryCatchLog-intro.html#workaround-2-withcallinghandlers-trycatch

Change-Id: I845dd8a0b9acf4d78340e563b2bbbbabd10b0d59
diff --git a/R/KorAPConnection.R b/R/KorAPConnection.R
index 69eecf7..3720c24 100644
--- a/R/KorAPConnection.R
+++ b/R/KorAPConnection.R
@@ -120,9 +120,10 @@
 
 #' @import keyring
 getAccessToken <- function(KorAPUrl) {
-    keyList <- withCallingHandlers(key_list(service = accessTokenServiceName),
+    keyList <- tryCatch(withCallingHandlers(key_list(service = accessTokenServiceName),
                                    warning = function(w) invokeRestart("muffleWarning"),
-                                   error = function(e) return(NULL))
+                                   error = function(e) return(NULL)),
+                          error = function(e) { })
   if (KorAPUrl %in% keyList)
     key_get(accessTokenServiceName, KorAPUrl)
   else