Better error message when uuid is not present in Project.toml

A nitpick, and clearly an instance of my own stupidity, however: While working through the MWE on my own codebase, I hit:

julia> create_app("MyApp", "MyAppCompiled")
ERROR: expected package to have a `name` and `uuid`

For whatever reason, I thought these should be arguments to create_app, which is totally wrong. It turns out that during generation of my package, no uuid was placed in the Project.toml, leading to this error message. (I now can't remember how I created it-it appears PkgTemplates now always adds a uuid so it's unclear how I got into this situation in the first place.) Nonetheless, it seems that the message could be improved to:

julia> create_app("MyApp", "MyAppCompiled")
ERROR: MyApp.jl/Project.toml has no `uuid`; this is required.

Of course willing to submit a PR as this seems like a straightforward change.