Rename some ClassObject/Provider-related methods

The terminology "field" is preferred over "key" for the components of a struct or struct-like object.

RELNOTES: None
PiperOrigin-RevId: 180269374
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/OutputGroupInfo.java b/src/main/java/com/google/devtools/build/lib/analysis/OutputGroupInfo.java
index 8c313d0..73c1f8b 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/OutputGroupInfo.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/OutputGroupInfo.java
@@ -252,7 +252,7 @@
   }
 
   @Override
-  public ImmutableCollection<String> getKeys() {
+  public ImmutableCollection<String> getFieldNames() {
     return outputGroups.keySet();
   }
 
@@ -281,8 +281,8 @@
     }
 
     @Override
-    public String getErrorMessageFormatForInstances() {
-      return "Output group %s not present";
+    public String getErrorMessageFormatForUnknownField() {
+      return "Output group '%s' not present";
     }
   }
 }