gobgp/cmd: add message gRPC connection error
gobgp/cmd: add message gRPC connection error

Adds a user-friendly error message on gRPC connection error.

Resolves: #1701

Signed-off-by: Paul Greenberg <f23aed6aca72079b9894fecd5d318c15ab02e1e5@outlook.com>

fujita/gobgp,a16/gobgp,a16/gobgp,osrg/gobgp,tamihiro/gobgp,a16/gobgp,osrg/gobgp,osrg/gobgp,a16/gobgp,a16/gobgp,j-kato/gobgp,j-kato/gobgp,a16/gobgp,osrg/gobgp,tamihiro/gobgp,tamihiro/gobgp,fujita/gobgp,fujita/gobgp,j-kato/gobgp,fujita/gobgp,a16/gobgp,tamihiro/gobgp