User-set custom warning messages

Currently a message like "maintenance/fuel is getting low/no power" is more like a death sentence - when it appears it is usually too late to fix the issue without death-spiraling happening. Moreover, it is a consequence of real issue (coal/oil supply interruption or some other). The good way to fix this would be an ability to create and configure custom warning messages for storages. So a user can set a condition for it to appear depending on the storage contents.
For example, a loose storage used for coal that supplies boilers of power plant. Everything is fine when it is full, but once it is not, u better know it right away before getting a blackout and the consequences of it. So enter storage UI, click a toggle button "Monitor contents: [yes]/[no] (default is [no])" and get a separate slidebar appear named "warn if contents are below this limit:" where u can set the limiter. So once storage has less material then limited, u get a warning message "[material] is low!" (Coal is low!)

Last update just implemented it. Not exactly the way it was proposed in this topic, but close enough to mark this entry as "Done".