Add ERROR state for capture agent

A capture agent should be able to indicate that it is in an ERROR state, i.e. unable to capture any recordings, for example because of a configuration error or capture hardware or stream being unavailable (e.g. an IP stream from a device that is down).

At present the CA can only show idle, unknown, or exit completely.

Currently supported states are:

/** The collection of all known states. TODO: Remove this when the states are replaced with enums */
List<String> KNOWN_STATES = Arrays.asList(new String[]

{ IDLE, SHUTTING_DOWN, CAPTURING, UPLOADING, UNKNOWN }

);