Better error messages, context

```
[00:00:00][ ][ main:core activity ] FAILURE Resolving elements
[00:00:01][ ][ main:core activity ] FAILURE Build

ref is not in the expected format
```

In the issue here I had a git `ref:` entry without a value. My fault. Still from the error message we can't even tell which of all the many bst files is at fault.

It would have been useful if it tells me which file is at fault; the line would be even better.

I'm putting it here instead of buildstream-plugins-community because looking at the `SourceError` class it doesn't have a location argument.

Of course, the conclusion could be that this ought to be included in the message. In such case we'd need a policy somewhere for such formatting.

Start by reviewing the SourceError class and the handling of the invalid git ref entry shown in the issue. Trace where the error loses its file and line context across the relevant bst files. Done means the error identifies the faulty file, ideally its line, with a consistent formatting policy.

Written by the indexing model from the issue text.