upgrades: check for ErrNoTools in tools migration

The tools storage migration step was failing if
there were no tools in the private provider storage.
This is fixed by checking for an additional error
in the upgrade step.

Unit test added, and live tested on AWS by uploading
custom tools to my own S3 bucket.

Fixes https://bugs.launchpad.net/juju-core/+bug/1385289
Merge pull request #975 from axw/lp1385289-upgrade-toolsstorage-notools

upgrades: check for ErrNoTools in tools migration

The tools storage migration step was failing if
there were no tools in the private provider storage.
This is fixed by checking for an additional error
in the upgrade step.

Unit test added, and live tested on AWS by uploading
custom tools to my own S3 bucket.

Fixes https://bugs.launchpad.net/juju-core/+bug/1385289