Studio: fix failing detection, add error message after reddit post

src/Logic/Tags/Tag.ts

@@ -170,8 +170,12 @@ export class Tag extends TagsFilter {
         return <any>this
     }
 
+    /**
+     * new Tag("panoramax", "").isNegative() // => true
+     * new Tag("x","y").isNegative() // => false
+     */
     isNegative(): boolean {
-        return false
+        return this.value === ""
     }
 
     visit(f: (tagsFilter: TagsFilter) => void) {

src/Models/ThemeConfig/Conversion/PrevalidateLayer.ts

@@ -69,7 +69,7 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
                     context
                         .enters("source", "osmTags")
                         .err(
-                            "The source states tags which give a very wide selection: it only uses negative expressions, which will result in too much and unexpected data. Add at least one required tag. The tags are:\n\t" +
+                            "The tags that will be used to load data from OpenStreetMap are all negative - this means that they all match something that _doesn't_ have a certain tag. For example, `key=` means anything without `key`. Did you perhaps mean to use `key~*`, meaning anything _with_ this key set? The tags are:\n\t" +
                                 osmTags.asHumanString(false, false, {})
                         )
                 }