CLI command to test connection to Ionic servers

Multiple commands like `ionic upload` or `ionic cordova resources` fail if the command line is not able to connect to Ionic Servers. Often adding proxies might solve the problem, but it would be nice to have a command you can tell people to check this. `ionic check connection` or something similar. (Of course this one command wouldn't justify adding the `check` command with a subcommand, but you get the idea)

Start by reviewing the existing `ionic upload` and `ionic cordova resources` commands to see how they connect to Ionic Servers and handle proxy settings. A completed change should provide a CLI command that clearly reports whether the connection works, including a useful failure result for users troubleshooting these commands.

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