Added support for step errors when executing backend (#817)

When executing a backend step, in case of failure of the specific step, the run is marked as errored but the step error is missing.

Added:
1. Log for the backend error (without trace)
2. Mark the step as errored with exit code 126 (Could not execute).

Co-authored-by: Zav Shotan <zshotan@bloomberg.net>
Co-authored-by: Anton Bracke <anton@ju60.de>

pipeline/backend/types/state.go

@@ -8,4 +8,6 @@ type State struct {
 	Exited bool `json:"exited"`
 	// Container is oom killed, true or false
 	OOMKilled bool `json:"oom_killed"`
+	// Container error
+	Error error
 }