Improve error message when network results do not exist

[alihamdan]

Consider this example

import roseau.load_flow as lf
en = lf.ElectricalNetwork.from_catalogue("LVFeeder00939", "summer")
en.res_loads  # <- error

Before:

RoseauLoadFlowException: Results for PowerLoad 'LVBus004111_production' are not available because the load flow has not been run yet. [load_flow_not_run]

After:

RoseauLoadFlowException: The load flow results are not available because the load flow has not been run yet. [load_flow_not_run]