[native] Alert user when UPDATE_USER_AVATAR call fails

Summary: Handled image selection/processing/upload in D7530, this handles the update avatar call in a slightly different way (as discussed in the that diff)

Test Plan:
Modify updateUserAvatarResponder to make sure request fails and observe that Alert appeared as expected:

Reviewers: ashoat, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7543