Improve failure message for backdeployed AsyncParsableCommand

[natecook1000]

When an executable with asynchronous commands is backdeployed, the compiler chooses the synchronous main() unless a minimum availability target is provided for the root command type. This changes the error message provided when the incorrect main() function is called to direct the tool's author to a correct solution.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test