Fix admin commands getting stuck

[AndrewFerr]

The two main reasons why admin commands got stuck are:

1. Calling yargs.parse multiple times apparently doesn't behave well when command handlers return a Promise:

Note: parse() should be called only once when command() is called with a handler returning a promise. If your use case requires parse() to be called several times, any asynchronous operation performed in a command handler should not result in the handler returning a promise.

2. When yargs.parse throws, or the callback given to it throws, yargs.argv is permanently set to a YError, and all future calls to yargs.parse throw that same error.

This PR cleans up some of our yargs usage & moves all asynchronicity of command callbacks outside of yargs.parse.

[jaller94]

Looks good.

[AndrewFerr]

The latest commit is needed to fix asynchronous commands not being awaited properly. It also moves error handling back into Main.ts to catch errors more generically & to minimize the size of the diff.