[POOL_UPGRADE] Incorrect message for POOL_UPGRADE with not unique name.

Overview:
Incorrect message for POOL_UPGRADE with not unique name.

Case 1:
Steps to Reproduce:
1. Open the CLI.
2. send POOL_UPGRADE command with valid parameters

send POOL_UPGRADE name=upgrade-oz1 version=0.3.163 sha256=aad1242 action=start schedule={'Gw6pDLhcBcoQesN72qfotTgFa7cbuqZpkX3Xo6pLhPhv': '2017-07-03T10:50:00.000000+00:00', '8ECVSk179mjsjKRLWiQtssMLgp6EPhWXtaYyStWPSGAb': '2017-07-03T10:55:00.000000+00:00', 'DKVxG2fXXTU8yT5N7hGEbXB3dfdAnYv1JczDUHpmDxya': '2017-07-03T11:00:00.000000+00:00', '4PS3EDQ3dW1tci1Bp6543CfuuebjFrg36kLAUcskGfaA': '2017-07-03T11:05:00.000000+00:00'} timeout=10

3. send the same POOL_UPGRADE again.

Actual Results:
Error message: "Pool upgrade failed: client request invalid: UnauthorizedClientRequest('TRUSTEE cannot do POOL_UPGRADE',)"
'TRUSTEE cannot do POOL_UPGRADE' is incorrect message, because TRUSTEE can do that.

Expected Results:
Error message should contain information about not unique name.