feat(validation): name a keyed collection as such when a list is given

tests/inputs/inputs_fixtures.yml

@@ -21,7 +21,7 @@ inputs_fixtures:
       name: web01
       users:
         - name: alice
-    expect: ["fixture.users: expected dict, got list"]
+    expect: ["fixture.users: expected dict keyed by name, got list"]
 
   - name: comma-string-for-list
     value:

tests/users/users_fixtures.yml

@@ -52,7 +52,7 @@ users_fixtures:
         - name: ops
           password: pw
     expect:
-      fail: "system.users: expected dict, got list"
+      fail: "system.users: expected dict keyed by name, got list"
 
   - name: list-rejected-precomputed
     precomputed: true
@@ -61,7 +61,7 @@ users_fixtures:
         - name: ops
           password: pw
     expect:
-      fail: "system.users: expected dict, got list"
+      fail: "system.users: expected dict keyed by name, got list"
 
   - name: scalar-entry-rejected
     system: