Catch and fix map random terrain problems (#259)

data/maps/Coastline.json

@@ -3,7 +3,7 @@
     "+": 1,
     "-": 50,
     "^": 6,
-    "r": 2,
+    "o": 2,
     "t": 14,
     "~": 20
   },

data/maps/Desert Valley.json

@@ -3,7 +3,7 @@
     "+": 40,
     "-": 15,
     "^": 1,
-    "r": 3,
+    "o": 3,
     "t": 4,
     "~": 4
   },

data/maps/Great Plains.json

@@ -2,7 +2,7 @@
   "terrains": {
     "-": 250,
     "^": 3,
-    "r": 3,
+    "o": 3,
     "t": 20,
     "~": 2
   },

data/maps/River Delta.json

@@ -2,7 +2,7 @@
   "terrains": {
     "-": 50,
     "^": 4,
-    "r": 1,
+    "o": 1,
     "t": 12,
     "~": 10
   },

data/maps/River.json

@@ -3,7 +3,7 @@
     "+": 1,
     "-": 20,
     "^": 4,
-    "r": 1,
+    "o": 1,
     "t": 6,
     "~": 6
   },

data/maps/Rolling Hills.json

@@ -2,7 +2,7 @@
   "terrains": {
     "-": 50,
     "^": 20,
-    "r": 1,
+    "o": 1,
     "t": 20,
     "~": 6
   },

data/maps/Swamp Forest.json

@@ -2,7 +2,7 @@
   "terrains": {
     "-": 40,
     "^": 1,
-    "r": 1,
+    "o": 1,
     "t": 25,
     "~": 6
   },

game/save/load.go

@@ -12,6 +12,7 @@ import (
 	"github.com/mlange-42/arche/ecs"
 	"github.com/mlange-42/tiny-world/game/comp"
 	"github.com/mlange-42/tiny-world/game/maps"
+	"github.com/mlange-42/tiny-world/game/terr"
 )
 
 func LoadWorld(world *ecs.World, folder, name string) error {
@@ -70,6 +71,20 @@ func ParseMap(mapStr string) (maps.Map, error) {
 			return maps.Map{}, fmt.Errorf("symbols must be single runes. Got '%s'", tStr)
 		}
 		sym := rn[0]
+		t, ok := terr.SymbolToTerrain[sym]
+		if !ok {
+			return maps.Map{}, fmt.Errorf("symbol not found: '%s'", tStr)
+		}
+		var ter terr.Terrain
+		if t.LandUse != terr.Air {
+			ter = t.LandUse
+		} else {
+			ter = t.Terrain
+		}
+		props := &terr.Properties[ter]
+		if props.TerrainBits.Contains(terr.CanBuy) || !props.TerrainBits.Contains(terr.CanBuild) {
+			return maps.Map{}, fmt.Errorf("terrain '%s' ('%s') is not a natural feature", props.Name, tStr)
+		}
 		for i := 0; i < cnt; i++ {
 			terrains = append(terrains, sym)
 		}