[test262] Always return a pseudo Error object

For unknown reasons I didn't have the energy to dig into, some Babel Error 
objects can't be written to so despite formatting the error message, the 
original one would still be used. To get around this I'm just constructing a 
fake object with a `name` and `message` so the correctly formatted messages are 
used.

compiler/forget/scripts/test262-preprocessor.js

@@ -16,13 +16,19 @@ module.exports = (test) => {
     }
   } catch (error) {
     // We use the `stderr` output to group errors so we can count them, so we need to dedupe errors
-    // that are the same but differ slightly
-    error.message = error.message.replace(/ \(\d+:\d+\)/, ""); // some errors report line numbers
-    error.message = error.message.replace(/\/.*\.js:\s/, ""); // babel seems to output filenames
+    // that are the same but differ slightly.
+    let { name, message } = error;
+    message = message.replace(/\/.*\.js:\s/, ""); // babel seems to output filenames
+    message = message.split(/\(\d+:\d+\)/)[0]; // some errors report line numbers and codeframes
+    message = message.trim();
+
+    // For unknown reasons I don't have the energy to dig into some Babel error instances can't be
+    // written to, so we construct a psedudo object here so the correctly formatted error messages
+    // are emitted
     test.result = {
-      stderr: `${error.name}: ${error.message}\n`,
+      stderr: `${name}: ${message}\n`,
       stdout: "",
-      error,
+      error: { name, message },
     };
   }