More verbose errors for NoManifestExceptions.

The old "manifest required for this command -- please run
init" is replaced by a more helpful message that lists the
command repo was trying to execute (with arguments) as well
as the str() of the NoManifestException. For example:

> error: in `sync`: [Errno 2] No such file or directory:
> 	'path/to/.repo/manifests/.git/HEAD'
> error: manifest missing or unreadable -- please run init

Other failure points in basic command parsing and dispatch
are more clearly explained in the same fashion.

Change-Id: I6212e5c648bc5d57e27145d55a5391ca565e4149