datasource-http-backend: improve error handling

[xnyo]

datasource-http-backend is using the same example error handling code as the datasource-basic.

Since the http backend example actually connects to an external service to fetch data, it would be nice to add proper error handling rather than an example one.

For example:

• 502 when remote service is unreachable
• 504 when the remote service times out
• ...