Allow emulators to work offline (MOTD fail)

[ericvera]

Happy to work on the design/messaging as this is occasionally blocking development due to power outages here in Puerto Rico.

Description

This change improve the offline experience for the CLI and unblocks usage of the emulator while offline.

As seen in bug #3916 when offline for more than a day since the last CLI usage, the CLI will attempt to download a MOTD from (https://firebase-public.firebaseio.com/cli.json). If it fails, it just shows an error about failing to fetch the URL.

With this change in the case of emulators:* commands the CLI will print a warning, but continue the execution. In the case of other commands it will print a more descriptive message (see sample outputs below).

Scenarios Tested

Tested running different commands both while online and while offline.

Sample Commands

firebase emulators:start --only auth,functions,firestore

Before:

eric@macbook sample-dir % firebase emulators:start --only auth,functions,firestore
⚠  emulators: You are not currently authenticated so some features may not work correctly. Please run firebase login to authenticate the CLI.

Error: Failed to make request to https://firebase-public.firebaseio.com/cli.json
i  emulators: Starting emulators: auth, functions, firestore
i  emulators: Shutting down emulators.

Error: No currently active project.
To run this command, you need to specify a project. You have two options:
- Run this command with --project <alias_or_project_id>.
- Set an active project by running firebase use --add, then rerun this command.
To list all the Firebase projects to which you have access, run firebase projects:list.
To learn about active projects for the CLI, visit https://firebase.google.com/docs/cli#project_aliases



Having trouble? Try firebase [command] --help

After: (new second emulators warning)

eric@macbook sample-dir % firebase emulators:start --only auth,functions,firestore
⚠  emulators: You are not currently authenticated so some features may not work correctly. Please run firebase login to authenticate the CLI.
⚠  emulators: Unable to fetch the CLI version check configuration, emulator functionality may be incorrect.
i  emulators: Starting emulators: auth, functions, firestore
[...emulators output successfull]

firebase login

Before:

eric@macbook sample-dir % firebase login                                          
i  Firebase optionally collects CLI and Emulator Suite usage and error reporting information to help improve our products. Data is collected in accordance with Google's privacy policy (https://policies.google.com/privacy) and is not used to identify you.

? Allow Firebase to collect CLI and Emulator Suite usage and error reporting information? (Y/n) 
Error: Failed to make request to https://firebase-public.firebaseio.com/cli.json

Having trouble? Try firebase [command] --help

After: (new error that explains how to get past the error)

eric@macbook sample-dir % firebase login
i  Firebase optionally collects CLI and Emulator Suite usage and error reporting information to help improve our products. Data is collected in accordance with Google's privacy policy (https://policies.google.com/privacy) and is not used to identify you.

? Allow Firebase to collect CLI and Emulator Suite usage and error reporting information? (Y/n) 
Error: Unable to fetch the CLI version check configuration. Make sure that you are online and try again.

Having trouble? Try firebase [command] --help

[bkendall]

Add a CHANGELOG entry that says:

- Catches errors when fetching CLI MOTD, allowing process to continue.

[ericvera]

Add a CHANGELOG entry that says:

- Catches errors when fetching CLI MOTD, allowing process to continue.

Added it to the latest changes.

[ericvera]

Accidentally closed the pull request.

[ericvera]

Re-ran local tests and a sample of commands (emulator and otherwise). They seem to all either proceed (e.g. emulators unless the .tar has not been downloaded yet) or exit gracefully when offline (e.g. functions:* commands).