BST_PROFILE should complain when the required profile doesn't exist

Summary

Currently, if you run buildstream with BST_PROFILE=nonexistent, everything will be fine, and you'll end up with no profiles at the end, which is surprising and unintuitive.

Steps to reproduce

Run BST_PROFILE=blah bst build

What is the current bug behavior?

No profile is created, nothing complains

What is the expected correct behavior?

We should at least get an warning, preferrably an error