Better error messages, context

[aleixpol]

[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.