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

project.py

@@ -2327,8 +2327,8 @@ class Project(object):
         path = os.path.join(self._project.worktree, '.git', HEAD)
       try:
         fd = open(path, 'rb')
-      except IOError:
-        raise NoManifestException(path)
+      except IOError as e:
+        raise NoManifestException(path, str(e))
       try:
         line = fd.read()
       finally: