Add support for custom errors in extensions SDK

[migmartri]

The extensions SDK should support custom errors that tweak two aspects

a) Error message exposure / logging / alerting
b) retry nature

For example

• validation error: A validation error should expose information to the end-user (the one executing the request) and should not get retried (in the case of being execution by the dispatcher)
• non-recoverable error: this error should not get retried by the scheduler.

This issue is about

### Tasks
- [ ] Defining and implementing the new set of custom errors and mechanisms
- [ ] Update the existing extensions to leverage these