Failing prepareInterstitial or prepareRewardVideoAd doesn't return error code

**Describe the bug**

Failing prepareInterstitial or prepareRewardVideoAd doesn't return error code.
It is currently very hard to determine cause for failure or rejection.

Most important codes for my needs are no fill rejections (ERROR_CODE_NO_FILL / GADErrorNoFill).

ERROR_CODE_MEDIATION_NO_FILL / GADErrorMediationNoFill will need special handling.

[Android](https://developers.google.com/admob/android/ad-load-errors)
[iOS](https://developers.google.com/admob/ios/ad-load-errors)

Error code values differ in Android and iOS.
So they need to be normalized.

**Expected behavior**
Error code should be returned in call.reject

Start by tracing the prepareInterstitial and prepareRewardVideoAd handlers on Android and iOS, focusing on their failure paths and call.reject behavior. Read the linked Android and iOS ad-load-error documentation to compare no-fill and mediation-no-fill cases. Done means platform error codes are normalized and the relevant code is returned through call.reject.

Written by the indexing model from the issue text.