minor error handling fixes (apache#25555)

sdks/go/pkg/beam/core/graph/fn.go

@@ -1099,13 +1099,13 @@ func validateSdfElementT(fn *Fn, name string, method *funcx.Fn, num int, startIn
 
 	for i := 0; i < num; i++ {
 		idx := i + startIndex
-		if method.Param[idx].T != processFn.Param[pos+i].T {
+		if got, want := method.Param[i+startIndex].T, processFn.Param[pos+i].T; got != want {
 			err := errors.Errorf("mismatched element type in method %v, param %v. got: %v, want: %v",
-				name, idx, method.Param[idx].T, processFn.Param[pos+i].T)
+				name, idx, got, want)
 			return errors.SetTopLevelMsgf(err, "Mismatched element type in method %v, "+
 				"parameter at index %v. Got: %v, Want: %v (from method %v). "+
 				"Ensure that element parameters in SDF methods have consistent types with element parameters in %v.",
-				name, idx, method.Param[idx].T, processFn.Param[pos+i].T, processElementName, processElementName)
+				name, idx, got, want, processElementName, processElementName)
 		}
 	}
 	return nil
@@ -1250,7 +1250,7 @@ func validateStatefulWatermarkSig(fn *Fn, numMainIn int) error {
 					"Ensure that all restrictions in an SDF are the same type.",
 					initialWatermarkEstimatorStateName, 1, method.Param[1].T, restT, createTrackerName)
 			}
-			if err := validateSdfElementT(fn, restrictionSizeName, method, numMainIn, 2); err != nil {
+			if err := validateSdfElementT(fn, initialWatermarkEstimatorStateName, method, numMainIn, 2); err != nil {
 				return err
 			}
 

sdks/go/pkg/beam/core/runtime/exec/datasink.go

@@ -77,7 +77,7 @@ func (n *DataSink) ProcessElement(ctx context.Context, value *FullValue, values
 		return err
 	}
 	if err := n.enc.Encode(value, &b); err != nil {
-		return errors.WithContextf(err, "encoding element %v with coder %v", value, n.enc)
+		return errors.WithContextf(err, "encoding element %v with coder %v", value, n.Coder)
 	}
 	byteCount, err := n.w.Write(b.Bytes())
 	if err != nil {