Update ES module loading error message

(cherry picked from commit 7f9b298)

Author: eleinadani

graal-js/src/com.oracle.truffle.js.test/src/com/oracle/truffle/js/test/interop/MLEBuiltinTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2021, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * The Universal Permissive License (UPL), Version 1.0
@@ -144,7 +144,7 @@ public void testUnknownSymbol() throws IOException {
             cx.eval(src);
         } catch (PolyglotException e) {
             failed = true;
-            assertEquals("Error: Custom ESM mapping not found for specifier: wrong", e.getMessage());
+            assertEquals("Error: Cannot load ES module: wrong", e.getMessage());
         } finally {
             cx.close();
         }

graal-js/src/com.oracle.truffle.js/src/com/oracle/truffle/js/runtime/JSRealm.java

@@ -2479,7 +2479,7 @@ public String getCustomEsmPathMapping(String refPath, String specifier) {
                     throw Errors.shouldNotReachHere(e);
                 }
             } else {
-                throw Errors.createError("Custom ESM mapping not found for specifier: " + specifier);
+                throw Errors.createError("Cannot load ES module: " + specifier);
             }
         }
         return null;