In some cases, due to promise chaining and error propagation, we were
calling registerError more than once for a given error. This would then
cause the overall callback for the send operation to be called with a
partial set of errors, as well as duplicates.
Note: we do need to find a way to attach identityKey to the
OutgoingIdentityKeyError in the case where it comes directly from the
encrypt() instead of our pre-key operations.
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