Update email handlers

[griffithcaleb]

🔹 JIRA Ticket(s) if any

✏️ Description

The updateEmail method in the RN SDK does pass down the onFailureHandler or onSuccessHandler that are available in
the Native SDKs -> expose these methods to the RN side.
Scenario and Use Case
When launching the app the an 'anonymous' user is created using the setUserId method. In the current implementation if a user logs in we will want to updateEmail or setEmail based on if the user's is currently registered with Iterable. Because there is no way to know this before, we will have to assume that we will want to update the user's email and associate a 'non-placeholder email address with that user'. However if we try and updateEmail on a user's email address that already exists in Iterable the updateEmail call will fail silently because the onFailureHandler has not been passed down.

Resolve a promise back across the RN bridge with the message from the failed API call. So that we can then
setEmail({theEmailTheUserHasLoggedinWith})

[roninopf]

Hi @griffithcaleb, thanks for the PR! Sorry for delays on reviewing and merging, it'll be on our list after the holiday break.