Catch attempts to compose a list

Attempting to compose a list property would result in an unhandled exception:

AttributeError: 'CommentedSeq' object has no attribute 'get'

We now at least detect the situation and produce an exception that the frontend will report neatly:

Error loading pipeline: element.bst [line 11 column 4]: Only values of type 'dict' can be composed.

I was getting this error from an attempt to conditionally extend the sources list:

sources:
  (?):
    arch == "x86_64":
      - url: http://example.com/x86_64

The correct way to do this is to move the conditional into the parent dict, e.g.:

(?):
  arch == "x86_64":
    sources:
      - url: https://example.com/x86_64

It would be nice if the error message could hint at how the user can do what they want, but it doesn't seem possible in this case.