cloud/datastore: fix Errorf verb
cloud/datastore: fix Errorf verb

Change-Id: Ie0fd8b10d532ebc001f1eaee713ce5cc5eaac42a

michael-grosshans/google-cloud-go,Maksadbek/gcloud-golang,Maksadbek/gcloud-golang,roth1002/gcloud-golang,hsinhoyeh/gcloud-golang,googleapis/google-cloud-go,ptomasroos/gcloud-golang,ptomasroos/gcloud-golang,michael-grosshans/google-cloud-go,roth1002/gcloud-golang,hsinhoyeh/gcloud-golang,blowmage/gcloud-golang,blowmage/gcloud-golang,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go