Add a warning in the admin when a required module is not installed

[cdfa]

Zotonic version

N/A

OS version

N/A

Description

• Motivation
  When a site requires an external zotonic module (such as mod_elasticsearch2) this needs to be cloned into the apps_user directory before the site will work.
  However, this step is often forgotten during setup and if you do, the problem is quite difficult to debug, because the server will reply 500, but without any errors in the console.
  Running z:compile(). does not always show an error either.

It's generally a problem that the server will reply with 500 without errors when not everything is compiled correctly. But I don't know if we can fix that in general.

• Proposal
  We should be able to detect it when a required module is not present in the filesystem.
  If this is the case, we should show a warning in the admin interface and the console, similar to Huge warning when required modules are inactive [Merge 0.x into master] #3045.

Current behavior

500 without any indication why

Expected behavior

500 with a warning saying why.

Config

N/A