Improve error communication from buildbox

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1201)
In GitLab by [[Gitlab user @traveltissues]](https://gitlab.com/traveltissues) on Nov 12, 2019, 10:31

There have been a few issues reported that were resolved by updating the versions of buildbox-common and buildbox-casd. This suggests that the errors from casd are not being clearly communicated and the solution is not being suggested to users clearly enough. We should review error handling and try to make this more clear. It might be possible to decide if the version of casd being used is suitable before falling over at broken api (for example), perhaps using a version string comparison during startup. If that is possible it must not impact startup time.

Start by tracing BuildStream's error handling and startup interaction with buildbox-common and buildbox-casd, focusing on failures caused by incompatible versions. Review how those errors are currently surfaced and whether version checking is feasible without affecting startup time. Done means users receive clear errors and actionable version guidance.

Written by the indexing model from the issue text.