Simplify wasmfs ErrnoError. NFC

This is the same change we already made to the old FS in emscripten-core#21149.

Author: sbc100

src/lib/libfs.js

@@ -59,7 +59,6 @@ FS.staticInit();
 #if FS_DEBUG
     trackingDelegate: {},
 #endif
-    ErrnoError: null, // set during init
     filesystems: null,
     syncFSRequests: 0, // we warn if there are multiple in flight at once
 #if expectToReceiveOnModule('logReadFiles')

src/lib/libwasmfs.js

@@ -21,9 +21,6 @@ addToLibrary({
   $wasmFSDevices: {},
   $wasmFSDeviceStreams: {},
 
-  $FS__postset: `
-FS.init();
-`,
   $FS__deps: [
     '$MEMFS',
     '$wasmFSPreloadedFiles',
@@ -74,10 +71,13 @@ FS.init();
 #endif
   ],
   $FS : {
-    init() {
-      FS.ensureErrnoError();
+    ErrnoError: class extends Error {
+      name = 'ErrnoError';
+      message = 'FS error';
+      constructor(code) {
+        this.code = code
+      }
     },
-    ErrnoError: null,
     handleError(returnValue) {
       // Assume errors correspond to negative returnValues
       // since some functions like _wasmfs_open() return positive
@@ -88,16 +88,6 @@ FS.init();
 
       return returnValue;
     },
-    ensureErrnoError() {
-      if (FS.ErrnoError) return;
-      FS.ErrnoError = /** @this{Object} */ function ErrnoError(code) {
-        this.errno = code;
-        this.message = 'FS error';
-        this.name = "ErrnoError";
-      }
-      FS.ErrnoError.prototype = new Error();
-      FS.ErrnoError.prototype.constructor = FS.ErrnoError;
-    },
     createDataFile(parent, name, fileData, canRead, canWrite, canOwn) {
       FS_createDataFile(parent, name, fileData, canRead, canWrite, canOwn);
     },

test/other/codesize/test_codesize_cxx_except.gzsize

@@ -1 +1 @@
-9269
+9271

test/other/codesize/test_codesize_cxx_mangle.gzsize

@@ -1 +1 @@
-9277
+9276