feat: Raise error if waved cannot find public/private dir #2130

[dbranley]

The PR fulfills these requirements: (check all the apply)

• It's submitted to the main branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. feat: Add a button #xxx, where "xxx" is the issue number).
• When resolving a specific issue, the PR description includes Closes #xxx, where "xxx" is the issue number.
• If changes were made to ui folder, unit tests (make test) still pass.
• New/updated tests are included

Closes #2130

As requested in the ticket, I have made changes so that an error is thrown when an invalid public or private directory is provided when starting the wave server. The following validation was added to server.go in the splitDirMapping() function:

    if _, err := os.Stat(xs[1]); os.IsNotExist(err){
        panic(fmt.Sprintf("directory does not exist: %s", xs[1]))
    }

I tested the change by temporarily modifying the run target in the Makefile to have various combinations of valid and invalid values for public-dir and public-dir. Here are screen shots of a couple examples where invalid directories were given:

And here are screen shots where valid directories are provided and I also included the browser in the screen shots to show that the server came up fine and the custom directory is accessible:

A couple things to note:

1. I assume testing this by running make run rather than using the waved command is sufficient. I normally run everything in my dev environment with make, but if you would rather I test using the waved command then let me know.
2. I did not test this on Windows, since I run on macos. I'm confident this should work fine on Windows, but wanted to make sure you were aware that I don't have a Windows machine to test this on.

Let me know if you have any questions or would like me to make any changes. Thanks!

[mturoci]

LGTM! Thanks @dbranley!