Improve composer error message

[slafleche]

I moved my vanilla-cli repo out of the synched docker folder. I forgot to run composer before running the build with the cli in my project. It gave me a composer error, but because it fed the error straight through, it gives the impression you need to run composer in your project folder, not in vanilla-cil. Anyways, I found the issue, but I figure a better error message could help future users of the tool.

You need to set up the project dependencies using Composer: