Handle logout having no candidates

pkg/cmd/auth/logout/logout.go

@@ -110,7 +110,9 @@ func logoutRun(opts *LogoutOptions) error {
 		}
 	}
 
-	if len(candidates) == 1 {
+	if len(candidates) == 0 {
+		return errors.New("no user accounts matched that criteria")
+	} else if len(candidates) == 1 {
 		hostname = candidates[0].host
 		username = candidates[0].user
 	} else if !opts.IO.CanPrompt() {

pkg/cmd/auth/logout/logout_test.go

@@ -240,6 +240,17 @@ func Test_logoutRun_tty(t *testing.T) {
 			},
 			wantErr: "not logged in as unknown-user on ghe.io",
 		},
+		{
+			name: "errors when user is specified but doesn't exist on any host",
+			opts: &LogoutOptions{
+				Username: "unknown-user",
+			},
+			cfgHosts: []hostUsers{
+				{"github.com", []user{"monalisa"}},
+				{"ghe.io", []user{"monalisa"}},
+			},
+			wantErr: "no user accounts matched that criteria",
+		},
 	}
 
 	for _, tt := range tests {