Fix bug where sse response hangs if handler threw an error.

[taeold]

When callable function handler throws an error, the existing implementation doesn't capture the error. Instead the error is throw, killing the process:

>  {"severity":"ERROR","message":"Unhandled error ReferenceError: datat is not defined\n    at /Users/[REDACTED_USER_NAME]/google/cf3-streaming-demo/functions/lib/index.js:90:9\n    at async /Users/[REDACTED_USER_NAME]/google/cf3-streaming-demo/functions/nod
e_modules/firebase-functions/lib/common/providers/https.js:544:26\n    at async runFunction (/Users/[REDACTED_USER_NAME]/google/firebase-tools/lib/emulator/functionsEmulatorRuntime.js:506:9)\n    at async runHTTPS (/Users/[REDACTED_USER_NAME]/google/firebase
-tools/lib/emulator/functionsEmulatorRuntime.js:531:5)\n    at async /Users/[REDACTED_USER_NAME]/google/firebase-tools/lib/emulator/functionsEmulatorRuntime.js:694:21"}
⚠  functions: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at new NodeError (node:internal/errors:405:5)
    at ServerResponse.setHeader (node:_http_outgoing:648:11)
    at ServerResponse.header (/node_modules/express/lib/response.js:795:10)
    at ServerResponse.contentType (/node_modules/express/lib/response.js:625:15)
    at ServerResponse.send (/Users/[REDACTED_USER_NAME]/google/firebase-tools/node_modules/express/lib/response.js:150:14)
    at /Users/[REDACTED_USER_NAME]/google/cf3-streaming-demo/functions/node_modules/firebase-functions/lib/common/providers/https.js:575:25
    at async runFunction (/Users/[REDACTED_USER_NAME]/google/firebase-tools/lib/emulator/functionsEmulatorRuntime.js:506:9)
    at async runHTTPS (/Users/[REDACTED_USER_NAME]/google/firebase-tools/lib/emulator/functionsEmulatorRuntime.js:531:5)
    at async /Users/[REDACTED_USER_NAME]/google/firebase-tools/lib/emulator/functionsEmulatorRuntime.js:694:21
⚠  Your function was killed because it raised an unhandled error.
i  functions: Finished "us-central1-genStreamError" in 5044.977417ms

I needed to make some changes to the MockResponse class since it incorrectly implemented how Response object actually works for node.js.

[inlined]

nit: consider String(toSend || "") if you want to both silence errors and have defined behavior at runtime

[taeold]

done.