fix: filesystem handler returned incorrect error when file not found

.gitignore

@@ -1 +1,2 @@
 dist
+cmd/gemini

filesystem.go

@@ -92,6 +92,10 @@ func FileSystemHandler(fs FileSystem) Handler {
 		}
 		f, err := fs.Open(r.URL.Path)
 		if err != nil {
+			if os.IsNotExist(err) {
+				NotFoundHandler().ServeGemini(w, r)
+				return
+			}
 			log.Warn("FileSystemHandler: file open failed", log.String("reason", err.Error()), log.String("path", r.URL.Path), log.String("url", r.URL.String()))
 			w.SetHeader(CodeTemporaryFailure, "file open failed")
 			return

filesystem_test.go

@@ -37,6 +37,15 @@ func TestFileSystemHandler(t *testing.T) {
 			expectedHeader: geminiSuccessHeader,
 			expectedBody:   "# /tests/a/index.gmi\n",
 		},
+		{
+			name: "non-existent files return a 51 status code",
+			url:  "/a/non-existent.gmi",
+			expectedHeader: Header{
+				Code: CodeNotFound,
+				Meta: "",
+			},
+			expectedBody: "",
+		},
 		{
 			name:           "a slash prefix is added if missing",
 			url:            "a/index.gmi",

server_test.go

@@ -149,7 +149,7 @@ func TestServer(t *testing.T) {
 			expectedBodyErr:   nil,
 		},
 		{
-			name:    "panics in handlers return a GCI error",
+			name:    "panics in handlers return a CGI error",
 			request: "gemini://sensible\r\n",
 			handler: func(w ResponseWriter, r *Request) {
 				panic("oops")