issues seen when upgrading OS from 12.04 to 14.04

[lavi chhabra]

I tried upgrading OS from 12.04 to 14.04, fetching software channel runs for a while post which it gives an error:

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

 This can be caused by:
 * Upgrading to a pre-release version of Ubuntu
 * Running the current pre-release version of Ubuntu
 * Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.17
ProcVersionSignature: Ubuntu 3.2.0-67.101-generic 3.2.60
Uname: Linux 3.2.0-67-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Tue Aug 12 23:53:06 2014
EcryptfsInUse: Yes
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1407866867
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 604
 com.ubuntu.update-manager window-width 600
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_IN:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_IN
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-08-12 (0 days ago)
VarLogDistupgradeTermlog: