Be more helpful when running with no asset dirs yet

[timriley]

Right now it errors with this message:

$ icelab-assets start --source-path=slices
Failed to compile.

Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
 - configuration.entry should be one of these:
   object { <key>: non-empty string | [non-empty string] } | non-empty string | [non-empty string] | function
   -> The entry point(s) of the compilation.
   Details:
    * configuration.entry should not be empty.
      -> Multiple entry bundles are created. The key is the chunk name. The value can be a string or an array.
    * configuration.entry should be a string.
      -> An entry point without name. The string is resolved to a module which is loaded upon startup.
    * configuration.entry should be an array:
      [non-empty string]
    * configuration.entry should be an instance of function
      -> A Function returning an entry object, an entry string, an entry array or a promise to these things.

error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

Ideally it'd (a) actually just run gracefully without any asset dirs yet, or (b) at least give a more helpful error message?

The scenario here is a real one: newly generated snowflakes app without any slice dirs yet (and therefore no assets). If I want to leave the assets server enabled in Procfile.dev, then it's not actually possible to start the naked app until assets are created.

[timriley]

This might actually duplicate #3