Handle nonexistent packages in Resolve.explicit

Fixes #810.
Factor out the calling of args.func into a function

This makes it possible to do the same catching of RuntimeError in external
packages like conda-build.
Actually run the command!
Don't print an error with conda config --get for other key types
Give a better error message for 404

Fixes #808.
Make sure the build number is an integer in the plan module
I want str.isdigit, not str.isnumeric, which is Python 3 only
Test that no stderr is produced with captured()
Add test for automatic grabbing of build number
Merge branch 'master' of github.com:conda/conda
Merge branch 'master' into rpath