Raise a warning instead of dying when the history file is invalid

Fixes #838.
Merge branch 'master' of github.com:conda/conda
Don't skip variables like HOMEPATH in launch()
Merge pull request #839 from lidavidm/fix_path

Don't skip variables like HOMEPATH in launch()
Merge branch 'master' into feature/issue_803

Conflicts:
	conda/misc.py