make parted tell the reason for non-optimal partition alignment

[Karl-Philipp Richter]

When partitions are created it happens that `parted` warns the user about non-optimal partition alignment. Rather than telling the user _that_ the partition is aligned non-optimal, it'd be very helpful to explain why the alignment is non-optimal or how such an optimal alignment would look like. Example:

    mkpart primary btrfs 0 100%
    Warning: The resulting partition is not properly aligned for best performance.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: parted 3.2-7ubuntu1
Uname: Linux 4.0.0-040000-generic x86_64
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Apr 20 20:29:48 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-02 (17 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: parted
UpgradeStatus: No upgrade log present (probably fresh install)