When net_tap driver can't initialize, emit human error

Introduces mirage.runtime library and corresponding Mirage_runtime module.
Fixes #164.
oasis regen
Bump to 2.0.2
improve the Mirage_runtime ocamldoc
Merge branch 'master' of github.com:mirage/mirage into tap-human-error

Conflicts:
	CHANGES
	lib/META
	setup.ml
Merge remote-tracking branch 'upstream/master' into tap-human-error
sync CHANGES
Merge pull request #330 from avsm/tap-human-error

Update to #327: Tap human error