presubmit: Check errorf as well (#1845)

Uppercase all these test errors as well. And extend the presubmit to
check for these in the future. Also do a slightly smarter grep to only
get t.<something>. as (because dump regexp) this also grep over non test
files.

plugin/cache/cache_test.go

@@ -175,7 +175,7 @@ func TestCache(t *testing.T) {
 		ok := i != nil
 
 		if ok != tc.shouldCache {
-			t.Errorf("cached message that should not have been cached: %s", state.Name())
+			t.Errorf("Cached message that should not have been cached: %s", state.Name())
 			continue
 		}