Improve how the ENSRainbow client handles network errors

[tk-o]

@lightwalker-eth suggested:

I think it would be nice to update the HealResponse data model so that these network errors could also be represented there and not just throw an exception. This is nice for people building UIs. For example it makes it easier to build a nice UI where you show an specialized UIs for errors such as "Connection Timeout" or "Server is unreachable"

[tk-o]

Related suggestion:

Suggest not to have any try / catch here around the actual API call itself and instead let any exception that might happen here flow through. Otherwise it becomes confusing if a 500 error is returned, since that is supposed to be for a server-side error.

Alternatively, I'm open to adding more distinct error codes for errors such as:

• Connection Lost
• Request Timeout

[djstrong]

The current implementation throw errors with network issues. I have added tests for this.