helpful output for create frontend name collision

[qrkourier]

The controller API at POST /frontend responds with a server error (500) if the requested frontend name already exists, and the CLI panics.

panic: [POST /frontend][500] createFrontendInternalServerError 

goroutine 1 [running]:
main.(*adminCreateFrontendCommand).run(0x0?, 0x0?, {0xc00036f8c0?, 0x0?, 0x0?})
        /home/runner/work/zrok/zrok/cmd/zrok/adminCreateFrontend.go:55 +0x226
github.com/spf13/cobra.(*Command).execute(0xc00036a500, {0xc00036f890, 0x3, 0x3})
        /home/runner/go/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:876 +0x67b
github.com/spf13/cobra.(*Command).ExecuteC(0x2b21aa0)
        /home/runner/go/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:990 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
        /home/runner/go/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:918
main.main()
        /home/runner/work/zrok/zrok/cmd/zrok/main.go:85 +0x25

The response should be a bad request (400). Ideally, the response body should include a message that admin create frontend can parse to provide helpful output to the CLI user.