all: Tweak error creation (#6391)

- In the few places where we wrap errors, use the new Go 1.13 "%w"
  construction instead of %s or %v.

- Where we create errors with constant strings, consistently use
  errors.New and not fmt.Errorf.

- Remove capitalization from errors in the few places where we had that.

meta/copyright_test.go

@@ -68,5 +68,5 @@ func checkCopyright(path string, info os.FileInfo, err error) error {
 		}
 	}
 
-	return fmt.Errorf("Missing copyright in %s?", path)
+	return fmt.Errorf("missing copyright in %s?", path)
 }