Improve check for invalid proxying mode. NFC (emscripten-core#26001)

Author: sbc100

src/jsifier.mjs

@@ -442,8 +442,8 @@ function(${args}) {
 
     const proxyingMode = LibraryManager.library[symbol + '__proxy'];
     if (proxyingMode) {
-      if (proxyingMode !== 'sync' && proxyingMode !== 'async' && proxyingMode !== 'none') {
-        throw new Error(`Invalid proxyingMode ${symbol}__proxy: '${proxyingMode}' specified!`);
+      if (!['sync', 'async', 'none'].includes(proxyingMode)) {
+        error(`JS library error: invalid proxying mode '${symbol}__proxy: ${proxyingMode}' specified`);
       }
       if (SHARED_MEMORY && proxyingMode != 'none') {
         const sync = proxyingMode === 'sync';

test/test_jslib.py

@@ -325,6 +325,16 @@ def test_jslib_i53abi_errors(self):
     err = self.expect_fail([EMCC, test_file('hello_world.c'), '-sDEFAULT_LIBRARY_FUNCS_TO_INCLUDE=jslibfunc', '--js-library', 'lib.js'])
     self.assertContained("error: JS library error: '__i53abi' only makes sense when '__sig' includes 'j' (int64): 'jslibfunc'", err)
 
+  def test_jslib_invalid_proxy_mode(self):
+    create_file('lib.js', r'''
+addToLibrary({
+  jslibfunc__proxy: 'foo',
+  jslibfunc: (x) => 42,
+});
+''')
+    err = self.expect_fail([EMCC, test_file('hello_world.c'), '-sDEFAULT_LIBRARY_FUNCS_TO_INCLUDE=jslibfunc', '--js-library', 'lib.js'])
+    self.assertContained("error: JS library error: invalid proxying mode 'jslibfunc__proxy: foo' specified", err)
+
   def test_jslib_legacy(self):
     create_file('lib.js', r'''
 mergeInto(LibraryManager.library, {