Report failure to user if user attempts to map logical node to multiple physical nodes.

FLUME-45 prevents logical nodes from being mapped to more than one physical node. Currently, the only indication this has happened is in the master's log files. Ideally this would return failure to the user when they attempt to execute mapping the command.

Suggested approaches to propagate information:
1) make addLogicalNode report information by either return boolean, true if success, false if failed, or (ii) throwing IllegalStateException. The latter approach seems more complicated, because all call sites may need to be careful to check all for this.
2) Modify CreateLogicalNodeForm.buildExecable..exec(...) so that it throws an IllegalStateException if addLogicalNode failed. The command/execable infrastructure is setup to captured exns and report them to the user.