Display error messages if pool delete throws error

Make error message display consistent across pool, share, service and
user pages.
Implement low level ntp service operations
Implement ntp service API
Add ntp to prepdb
Fixes #232

Add cli support for ntp
Display error on pool delete error

Make error display consistent for add pool and add share
Merge branch 'issue#227_pool_delete_error' of git://github.com/sujeetsr/rockstor-core into sujeetsr-issue#227_pool_delete_error
Merge pull request #237 from schakrava/issue#232_ntp

Issue#232 ntp
Fixes #226

Supply pool name with disks api so front end doesn't have to make extra calls.
Generate correct pool url for the disks table