Surface 401 from instance detection so the user can supply credentials

When an instance sits behind a reverse proxy that requires HTTP Basic Auth,
every detection probe (/info, /api/v1/config, /api/v1/stats, /healthcheck,
/config) returns 401 before reaching the real backend, so the type cannot be
identified. Re-throw APIError.unauthorized from each probe instead of
swallowing it, and have detectWithResult convert the first 401 it sees into
DetectionError.basicAuthRequired. Add a basicAuthHeader parameter so the
caller can retry detection after the user provides credentials; if a retry
also returns 401, surface basicAuthInvalid instead.

Yattee/Localizable.xcstrings

@@ -14593,6 +14593,28 @@
         }
       }
     },
+    "sources.error.basicAuthInvalid" : {
+      "comment" : "Error when HTTP Basic Auth credentials are rejected by the server",
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Invalid HTTP Basic Auth credentials. Check the username and password."
+          }
+        }
+      }
+    },
+    "sources.error.basicAuthRequired" : {
+      "comment" : "Error when an instance is fronted by HTTP Basic Auth and credentials are required to detect its type",
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "This instance requires HTTP Basic Auth. Enter a username and password to continue."
+          }
+        }
+      }
+    },
     "sources.error.couldNotDetect" : {
       "comment" : "Error when source type could not be detected",
       "localizations" : {