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.