RepoCommand: Add error to ManifestErrorException

RepoCommand: Add error to ManifestErrorException

RepoCommand wraps errors in the manifest in a ManifestErrorException
with a fixed message ("Invalid manifest"). Callers like supermanifest
plugin cannot return a meaningful error to the client without digging
into the cause chain.

Add the actual error message to the ManifestErrorException, so callers
can rely on #getMessage() to see what happens.

Change-Id: I18be17fb5e4aaaf4f11ebd627580a91fe330eaca