Don't mutate error message (#58452) (#58477)

* fix: 🐛 don't mutate error object in-place

This avoids mutating error thrown by an expression function in-place.
The error might not even be an object, in which case mutating it will
throw.

* test: 💍 capture in test that thrown error is not mutated

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

src/plugins/expressions/common/execution/execution.test.ts

@@ -630,6 +630,7 @@ describe('Execution', () => {
           },
         });
         expect(node2.debug?.rawError).toBeInstanceOf(Error);
+        expect(node2.debug?.rawError).toEqual(new Error('foo'));
       });
 
       test('sets .debug object to expected shape', async () => {

src/plugins/expressions/common/execution/execution.ts

@@ -230,8 +230,8 @@ export class Execution<
         input = output;
       } catch (rawError) {
         const timeEnd: number = this.params.debug ? performance.now() : 0;
-        rawError.message = `[${fnName}] > ${rawError.message}`;
         const error = createError(rawError) as ExpressionValueError;
+        error.error.message = `[${fnName}] > ${error.error.message}`;
 
         if (this.params.debug) {
           (link as ExpressionAstFunction).debug = {