Propagate framework errors to user application

[gregorygaines]

Currently, errors that happen in the framework express layer are passed to the default express error handler. The default error handler returns the error to the client with a stack trace if in non-prod enviornments. It would be useful for certain cases for the user to have context of what happened during a request instead of the user being bypassed entirely. Instead, if the user has an error handler middleware installed in their application, we should pass the framework error to the user's error handler.

For example, if the framework gets a request with a bad json body.

const app = express():

app.post("/", (req, res) => {
 ...
});

// User error handler
app.use((err, req, res, next) => {
  logger.log(err);
  res.send("Caught error!");
});

functions.http("function, app);

// Post request with bad JSON
http.post("/", "{"id": "Hello}");

The framework responds with the following not very helpful message from the Express default error handler:

SyntaxError: Expected double-quoted property name in JSON at position 20 (line 3 column 1)
    at JSON.parse (<anonymous>)
    at parse (functions-framework-nodejs/node_modules/body-parser/lib/types/json.js:92:19)
    at functions-framework-nodejs/node_modules/body-parser/lib/read.js:128:18
    at AsyncResource.runInAsyncScope (node:async_hooks:211:14)
    at invokeCallback (functions-framework-nodejs/node_modules/raw-body/index.js:238:16)
    at done (functions-framework-nodejs/node_modules/raw-body/index.js:227:7)
    at IncomingMessage.onEnd (functions-framework-nodejs/node_modules/raw-body/index.js:287:7)
    at IncomingMessage.emit (node:events:518:28)
    at endReadableNT (node:internal/streams/readable:1698:12)
    at process.processTicksAndRejections (node:internal/process/task_queues:90:21)

There should be an option to propagate errors to the user client applcation, so their error handler is triggered and can preform logging, mitigations, or any other action the user wants instead of ignoring and leaving their application in the dust.

I've created #685 to address this issue.