juju should provide more useful error when no ssh keys avaliable

[Dave Walker]

Currently, if no ~/.ssh/id_* is available, juju provides output similar to:

$ juju bootstrap
2011-09-28 08:24:04,511 INFO Bootstrapping environment 'sample' (type: ec2)...
SSH authorized/public key not found.
2011-09-28 08:24:05,063 ERROR SSH authorized/public key not found.

This error could be more useful, suggesting the next steps to create a key.

As reported, https://answers.launchpad.net/ubuntu/+source/ensemble/+question/172588 .. and a user questioning this with me today.

Thanks :)