feat: Add client_report discard reasons send_error & internal_sdk_error (#830)

Author: mydea

develop-docs/sdk/client-reports.mdx

@@ -97,6 +97,8 @@ The following discard reasons are currently defined for `discarded_events`:
 - `sample_rate`: an event was dropped because of the configured sample rate.
 - `before_send`: an event was dropped in `before_send`
 - `event_processor`: an event was dropped by an event processor; may also be used for ignored exceptions / errors
+- `send_error`: an event was dropped because of an error when sending it (eg: 400 response)
+- `internal_sdk_error`: an event was dropped due to an internal SDK error (eg: web worker crash)
 
 In case a reason needs to be added,
 it also has to be added to the allowlist in [snuba](https://github.com/getsentry/snuba/blob/4e7cfdddcf7b93eacb762bc74ca2461cec9464e5/snuba/datasets/outcomes_processor.py#L24-L34).