client/status: notify user if size estimation is imprecise

There's plenty of room for improvement here.
For example, detect if we're past the last step without size estimation
and compute the remaining sum of bytes to be replicated from there on.

replication/report/replication_report.go

@@ -91,19 +91,21 @@ type StepInfo struct {
 	BytesReplicated int64
 }
 
-func (a *AttemptReport) BytesSum() (expected, replicated int64) {
+func (a *AttemptReport) BytesSum() (expected, replicated int64, containsInvalidSizeEstimates bool) {
 	for _, fs := range a.Filesystems {
-		e, r := fs.BytesSum()
+		e, r, fsContainsInvalidEstimate := fs.BytesSum()
+		containsInvalidSizeEstimates = containsInvalidSizeEstimates || fsContainsInvalidEstimate
 		expected += e
 		replicated += r
 	}
-	return expected, replicated
+	return expected, replicated, containsInvalidSizeEstimates
 }
 
-func (f *FilesystemReport) BytesSum() (expected, replicated int64) {
+func (f *FilesystemReport) BytesSum() (expected, replicated int64, containsInvalidSizeEstimates bool) {
 	for _, step := range f.Steps {
 		expected += step.Info.BytesExpected
 		replicated += step.Info.BytesReplicated
+		containsInvalidSizeEstimates = containsInvalidSizeEstimates || step.Info.BytesExpected == 0
 	}
 	return
 }