Implement proper exception handling throughout the project

[a-wai]

Over the past few weeks, we rushed to get features working on KCIv2 and consciously ignored some engineering best practices while doing so (which is OK and is understandable given the context).

At this point, we have frequent crashes due to not handling exceptions properly, e.g. kernelci/kernelci-pipeline#468 and kernelci/kernelci-pipeline#475 being only recent examples. This affects development efforts as such crashes require immediate attention to get the system up and running again, while delaying important results.

Exceptions should be properly handled in every part of the code that could fail (including handling KeyError for every dictionary access that isn't properly guarded by a type/key existence check).