gracefully handle the REPL being missing

[afeld]

Closes #48. Builds on #44 - diff.

If the REPL corresponding to the desired service can't be found, fall back to -no-client behavior.

$ which psql
psql not found
$ cf connect-to-service myapp mydb
Finding the service instance details...
Setting up SSH tunnel...
SSH tunnel created.
Executable `psql` not found.
Falling back to `-no-client` behavior.
Skipping call to client CLI. Connection information:

Host: localhost
Port: 58923
Username: myuser
Password: mypass
Name: dbname

To connect:

    PGPASSWORD=mypass psql -h localhost -p 58923 dbname myuser

Leave this terminal open while you want to use the SSH tunnel. Press Control-C to stop.