feat(util:context) add error logging to context

@jbenet 
@whyrusleeping 

This commit...

is a second attempt at: https://github.com/jbenet/go-ipfs/pull/68

partially addresses: https://github.com/jbenet/go-ipfs/issues/66

is the result of discussion at: https://gist.github.com/perfmode/f2951c1ed3a02c484d0b