Relay update warnings to the user.

[pfrenssen]

Fixes #5335

How to test:

Add the following line to core/modules/node/node.install:

function node_update_8000() {}

Run the drush updatedb:status command. A warning should be shown:

$ drush updatedb:status
 [warning] node: <em>node</em> module cannot be updated. It contains an update numbered as 8000 which is reserved for the earliest installation of a module in Drupal 8.x, before any updates. In order to update <em>node</em> module, you will need to install a version of the module with valid updates.
 [success] No database updates required.

[weitzman]

LGTM. Up to you if you want to make a test for this.

[pfrenssen]

Thanks! This is an edge case which will only affect a minority of users, so I think a test does not bring much value.

It only happens if a developer makes a mistake in the naming of an update hook. Still a good idea to show the warnings in this case :)