invalid character '<' error when Astarte/cluster does not reply as expected

[Pavinati]

When astartectl fails to parse data from the server, the following error is returned

invalid character '<' looking for beginning of value

for example by using the wrong URL in the parameters

astartectl housekeeping realms list --housekeeping-url https://api.example.com/wrong/path

Returning a more user friendly message would help identifying the problem.