Detect if in `zsh` subshell invoked via `$(maws)` and return error message

[gene1wood]

Given that $(maws) doesn't work in zsh (causes double requests #134), is there any way to detect that maws is running in a zsh subshell that was invoked with $()? If so, we should detect that and error out.

If not, then we can close this issue out.