[Elastic Agent] Add process error handling guidelines

x-pack/elastic-agent/GUIDELINES.md

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+# Guidelines
+
+This document contains architecture details around Elastic Agent and guidelines on how new inputs and processes should be built.
+
+## Processes running as service and error handling
+
+All the processes started by Elastic Agent are running as service. Each service is expected to handle local errors on its own and continue working. A process should only fail on startup if a non valid configuration is passed in. As soon as a process is running and partial updates to the config are made without restart, the service is expected to keep running but report the errors.
+
+A service that needs to do setup tasks on startup is expected to retry until it succeeds and not error out after a certain timeout.