serve s3: make errors in --s3-auth-key fatal - fixes #9044

Previously if auth keys were provided without a comma then rclone
would only log an INFO message which could mean it went on to serve
without any auth.

The parsing for environment variables was changed in v1.70.0 to make
them work properly with multiple inputs. This means the input is
treated like a mini CSV file which works well except in this case when
the input has commas. This meant `user,auth` without quotes is treated
as two key pairs `user` and `quote`. The correct syntax is
`"user,auth"`. This updates the documentation accordingly.

cmd/serve/s3/server.go

@@ -70,6 +70,11 @@ func newServer(ctx context.Context, f fs.Fs, opt *Options, vfsOpt *vfscommon.Opt
 		w.s3Secret = getAuthSecret(opt.AuthKey)
 	}
 
+	authList, err := authlistResolver(opt.AuthKey)
+	if err != nil {
+		return nil, fmt.Errorf("parsing auth list failed: %q", err)
+	}
+
 	var newLogger logger
 	w.faker = gofakes3.New(
 		newBackend(w),
@@ -77,7 +82,7 @@ func newServer(ctx context.Context, f fs.Fs, opt *Options, vfsOpt *vfscommon.Opt
 		gofakes3.WithLogger(newLogger),
 		gofakes3.WithRequestID(rand.Uint64()),
 		gofakes3.WithoutVersioning(),
-		gofakes3.WithV4Auth(authlistResolver(opt.AuthKey)),
+		gofakes3.WithV4Auth(authList),
 		gofakes3.WithIntegrityCheck(true), // Check Content-MD5 if supplied
 	)
 
@@ -92,7 +97,7 @@ func newServer(ctx context.Context, f fs.Fs, opt *Options, vfsOpt *vfscommon.Opt
 		w._vfs = vfs.New(f, vfsOpt)
 
 		if len(opt.AuthKey) > 0 {
-			w.faker.AddAuthKeys(authlistResolver(opt.AuthKey))
+			w.faker.AddAuthKeys(authList)
 		}
 	}