Automatically skip packages that have dependencies that can't be found

If you try to explicitly install such a package, it will still give an error
message. This fixes an issue where packages that aren't what you are actually
interested in a repo prevent installation from working because they include
dependencies that aren't found.

Fixes #599.
Give a better error message for not found packages
Fix the tests
Merge branch 'master' into depsnotfound
Raise correct exception that is caught in later code
Merge branch 'master' into depsnotfound

Conflicts:
	conda/resolve.py
Merge pull request #620 from conda/depsnotfound

Automatically skip packages that have dependencies that can't be found