buildstream/_pipeline.py: Simplify error message for assert_consistent()

Currently, `Pipeline.assert_consistent()` prints an error message like
the one below when certain elements have inconsistent sources:

```
...
      Element: hello.bst is inconsistent
        Source tar source at hello.bst [line 16 column 2] is missing ref
...
```

Drop the word "Source" from the beginning of the message as
"Source <source-kind> source ..." is kind of awkward to read.