Handle the error better when all the hosts are excluded using '-x' command

[amrutagokhale]

On a four node cluster with master, slave1, slave2 and slave3, if I run a presto-admin command with '-x' and mention all the hosts, the error message shown to the user is not helpful at all from the user's perspective. For example:

$./prestoadmin/presto-admin connector add -x master,slave1,slave2,slave3
Deploying hive.properties connector configurations on: None 

Fatal error: Needed to prompt for the target host connection string (host: None), but input would be ambiguous in parallel mode

Aborting.

$./prestoadmin/presto-admin configuration show -x master,slave1,slave2,slave3

Fatal error: Needed to prompt for the target host connection string (host: None), but input would be ambiguous in parallel mode

Aborting.

A better error message should be along the lines of: "You have chosen to exclude all the nodes for running this command, hence this operation is essentially a no-op"

I suspect that this is true for all presto-admin commands that support '-x' option, though I haven't tested it for all the commands.

The fix may not be as simple as modifying an existing error message. You may have to dig deeper to fix it.