provide pathes when encountering permission errors

[Karl-Philipp Richter]

If an invokation of `bundle install` fails due to permission error(s) feedback in the form of

    $ bundle install
    error: cannot open .git/FETCH_HEAD: Permission denied

    Retrying git fetch --force --quiet --tags "/home/richter/.bundler/cache/git/vagrant-spec-7f2ea299b3766e9a077e5bd825aa24cac5a3ef16" due to error (2/3): Bundler::Source::Git::GitCommandError Git error: command `git fetch --force --quiet --tags "/home/richter/.bundler/cache/git/vagrant-spec-7f2ea299b3766e9a077e5bd825aa24cac5a3ef16"` in directory /var/lib/gems/2.1.0/bundler/gems/vagrant-spec-1df5a3af81cb has failed.
    If this error persists you could try removing the cache directory '/home/richter/.bundler/cache/git/vagrant-spec-7f2ea299b3766e9a077e5bd825aa24cac5a3ef16'
    error: cannot open .git/FETCH_HEAD: Permission denied

    Retrying git fetch --force --quiet --tags "/home/richter/.bundler/cache/git/vagrant-spec-7f2ea299b3766e9a077e5bd825aa24cac5a3ef16" due to error (3/3): Bundler::Source::Git::GitCommandError Git error: command `git fetch --force --quiet --tags "/home/richter/.bundler/cache/git/vagrant-spec-7f2ea299b3766e9a077e5bd825aa24cac5a3ef16"` in directory /var/lib/gems/2.1.0/bundler/gems/vagrant-spec-1df5a3af81cb has failed.
    If this error persists you could try removing the cache directory '/home/richter/.bundler/cache/git/vagrant-spec-7f2ea299b3766e9a077e5bd825aa24cac5a3ef16'
    error: cannot open .git/FETCH_HEAD: Permission denied

    Git error: command `git fetch --force --quiet --tags "/home/richter/.bundler/cache/git/vagrant-spec-7f2ea299b3766e9a077e5bd825aa24cac5a3ef16"`
    in directory /var/lib/gems/2.1.0/bundler/gems/vagrant-spec-1df5a3af81cb has failed.
    If this error persists you could try removing the cache directory
    '/home/richter/.bundler/cache/git/vagrant-spec-7f2ea299b3766e9a077e5bd825aa24cac5a3ef16'

is given where `.git/FETCH_HEAD` must belong to another directory than the one mentioned with full path because removing the directory as suggested doesn't help, but even if it'd belong to the same directory giving the full path of `.git/FETCH_HEAD` would be helpful for debugging.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: ruby 1:2.1.0.4ubuntu1
Uname: Linux 4.0.0-040000-generic x86_64
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Tue Apr 28 01:03:52 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-02 (25 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: ruby-defaults
SystemImageInfo:
 current build number: 0
 device name:
 channel: daily
 last update: Unknown
UpgradeStatus: No upgrade log present (probably fresh install)

[Karl-Philipp Richter (krichter722)]

Or is `bundle` simply no asking for root permissions if necessary like it suggest ("Don't run Bundler as root. Bundler can ask for sudo if it is needed, and installing your bundle as root will break this application for all non-root users on this machine.") when run with `sudo`?