fix(kæde): stald-mangel forklares rigtigt og beskeder nævner kæden

Køb af en rest efter gårdens stalde giver nu spu_chain_problem_barn (køb resten
eller flere dyr) i stedet for 'byg en fabrik'. Engangsbeskeden får kædens navn
foran, så to kæder der mangler samme vare kan skelnes.

docs/USER_MANUAL.md

@@ -670,13 +670,14 @@ If a link in a chain is missing, the mod explains why and what you can do. The t
 - **In the Chains tab**: fold the chain out; the reason is shown in red below the red goods row.
 - **In Needs attention**: one red line per chain, for example *Bread is missing a link: Flour*. At most 2 chains
   are shown; the rest are in the Chains tab. Clicking opens the Chains tab with that chain folded out.
-- **As an on-screen message**: the first time a new link goes missing. The same message only returns after the
+- **As an on-screen message**: the first time a new link goes missing, with the chain's name in front (for example *Bread: …*). The same message only returns after the
   problem has gone away and come back. No messages during the first 2 minutes after loading or while fields are
   being scanned.
 
 The messages:
 
 - *None of your factories makes Flour — build one or buy it*: no factory, barn or field supplies the goods.
+- *Barns make too little Milk — buy the rest or more animals*: the barn makes the goods, but too little.
 - *Mill can only make 800 of 1.0k/mo*: the factory cannot keep up (bottleneck).
 - *Mill (Flour) was turned off by you — turn it on or pause the chain*: you switched the production off yourself.
 - *Loop around Flour — the chain cannot be calculated*: the goods are needed to make themselves.