Report all parse errors in one pass

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/459)
In GitLab by [[Gitlab user @tristanvb]](https://gitlab.com/tristanvb) on Jul 6, 2018, 12:27

## Summary

[//]: # (Summarize the bug encountered concisely)

It would be desirable if at load time, BuildStream would not abort the load process immediately when an error is encountered, but instead continue to collect as many errors with the yaml as rationally possible, and report all the errors at once.

## Other relevant information

[//]: # (Add the BuildStream version. Substitute below the "x" by "1", "2" or the right version. Add more than one version if necessary)

* BuildStream version affected: /milestone %BuildStream_v1.x

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Start with the load-time YAML parsing behavior described in this issue and compare it with the original GitLab issue for context. Determine which parse errors can be collected in one pass without aborting, and consider the work complete when all rationally discoverable errors are reported together.

Written by the indexing model from the issue text.