Add a configurable port number and error handling to the SFTP plugin

[ryanslatten]

This pr adds a port setting to connect to the sftp server instead of only using the default port 22. It also adds connection test so that you can tell if the connection works as well as error handling for failed connections.

Testing steps:

1. Test connecting to an SFTP server that doesn't use the standard 22 port and verify that it connects by clicking the "Test Connection" button
2. Attempt to connect to a SFTP server with incorrect credentials and verify that you can an understandable error and that the test connection fails.

[ryanslatten]

This PR is waiting on #368