better error for introspection unreachable endpoint

[EverlastingBugstopper]

if you try to rover subgraph introspect http://localhost:4000 when it's not running, you should get a semi-nice error message instead of what we currently send which is this:

error: encountered an error while sending a request
        This error was unexpected! Please submit an issue with any relevant details about what you were trying to do: https://github.com/apollographql/rover/issues/new

this error is a bit heavy handed so we should likely fix it up in other places it might appear as well

[lrlna]

Ohhhh, Adding this to May 11. This should be a quick fix, and would be good to be done for GA.