ios/android: expose connectivity errors via EnvoyError enumerated code

When requests time out due to network connectivity or being offline entirely, Envoy Mobile currently calls `onError` with an `EnvoyError` containing a `0` status code and `upstream request timeout` message.

In order to better communicate _which error_ occurred to end consumers, we can replace the `code` with an enumeration containing human-readable error reasons.

For example:

```swift
enum EnvoyErrorCode: Int {
case unknown = 0
case upstreamTimeout = 1
case dnsFailure = 2
...
}
```