We don't appear to be retrying `/sendToDevice` requests

Nobody has claimed this yet.

I don't think we're retrying failed `/sendToDevice` requests. [This comment](https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/algorithms/megolm.ts#L1634) seems to agree.

This is especially bad when e.g. sharing room keys in `encryptAndSendKeysToDevices` since we might end up [marking a key as sent](https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/algorithms/megolm.ts#L634) to a device when in reality the request failed. Though I'm not sure whether the `then(...)` is executed if the request fails or not.

All call sites:

- https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/algorithms/megolm.ts#L634
- https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/algorithms/megolm.ts#L686
- https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/algorithms/megolm.ts#L780
- https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/algorithms/megolm.ts#L1552
- https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/algorithms/megolm.ts#L1635
- https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/algorithms/megolm.ts#L1823
- https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/index.ts#L3313
- https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/OutgoingRoomKeyRequestManager.ts#L481
- https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/SecretStorage.ts#L402
- https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/SecretStorage.ts#L423
- https://github.com/matrix-org/matrix-js-sdk/blob/96e8f65af7429339dbed4bc8063b893b4a0966ce/src/crypto/SecretStorage.ts#L525

Start by inspecting the `/sendToDevice` implementation and the listed call sites in `src/crypto/algorithms/megolm.ts`, `src/crypto/index.ts`, `src/crypto/OutgoingRoomKeyRequestManager.ts`, and `src/crypto/SecretStorage.ts`. Trace how rejected requests reach each `then(...)`, especially `encryptAndSendKeysToDevices`. Done means failed requests are retried consistently and a key is not marked as sent when delivery failed.

Written by the indexing model from the issue text.