Improve `is_connected` error handling

Related to #7.

Right now, a flow controller will return `False` if initialized as a `FlowMeter`. We should consider a more verbose approach to `is_connected` covering the following scenarios:

 * Nothing's connected
 * Nothing's communicating
 * Something's communicating, but it's not an Alicat
 * An Alicat is communicating, but it's not a `FlowMeter` / `FlowController`