A plugin missing files it claims to contain doesn't get reported #5840

Current Behavior

The following applies to plugins. It could easily apply to nodes as well - need to check that path when fixing.

A plugin's package.json includes a declaration of plugin files:

"node-red": {
        "plugins": {
            "foo": "lib/foo.js"
        }
    },

If lib/foo.js doesn't exist, no errors are reported; the plugin silently fails to load.

Technically, it isn't silent - as this was the root cause of the odd errors reported in #5805.

The fix applied for #5805 hides the odd errors - but we should be reporting the missing file in the log.

Expected Behavior

A clear log message if a plugin declares a file that doesn't exist.

Steps To Reproduce

Create a simple plugin with a missing file.

Example flow

paste your flow here

Environment

• Node-RED version:
• Node.js version:
• npm version:
• Platform/OS:
• Browser: