builder/amazon/common: Log error messages from goamz on state change errors

/cc @jmassara - easy stuff :)

StepModifyAMIAttributes

/cc @jmassara - I pulled out the AMI stuff into a common config
struct and also added in the new template stuff (that didn't exist
when you made the pull, so not your fault! :))

/cc @jmassara

/cc @jmassara

Conflicts:
	website/source/docs/builders/amazon-instance.html.markdown