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.

test/sync_test.go

@@ -265,7 +265,7 @@ func scSyncAndCompare(p []*rc.Process, expected [][]fileInfo) error {
 			return err
 		}
 		if err := compareDirectoryContents(actual, expected[0]); err != nil {
-			return fmt.Errorf("%s: %v", dir, err)
+			return fmt.Errorf("%s: %w", dir, err)
 		}
 	}
 
@@ -276,7 +276,7 @@ func scSyncAndCompare(p []*rc.Process, expected [][]fileInfo) error {
 				return err
 			}
 			if err := compareDirectoryContents(actual, expected[1]); err != nil {
-				return fmt.Errorf("%s: %v", dir, err)
+				return fmt.Errorf("%s: %w", dir, err)
 			}
 		}
 	}
@@ -288,7 +288,7 @@ func scSyncAndCompare(p []*rc.Process, expected [][]fileInfo) error {
 				return err
 			}
 			if err := compareDirectoryContents(actual, expected[2]); err != nil {
-				return fmt.Errorf("%s: %v", dir, err)
+				return fmt.Errorf("%s: %w", dir, err)
 			}
 		}
 	}

test/util.go

@@ -330,7 +330,7 @@ func compareDirectories(dirs ...string) error {
 		for i := 1; i < len(res); i++ {
 			if res[i] != res[0] {
 				close(abort)
-				return fmt.Errorf("Mismatch; %#v (%s) != %#v (%s)", res[i], dirs[i], res[0], dirs[0])
+				return fmt.Errorf("mismatch; %#v (%s) != %#v (%s)", res[i], dirs[i], res[0], dirs[0])
 			}
 		}
 
@@ -381,7 +381,7 @@ func compareDirectoryContents(actual, expected []fileInfo) error {
 
 	for i := range actual {
 		if actual[i] != expected[i] {
-			return fmt.Errorf("Mismatch; actual %#v != expected %#v", actual[i], expected[i])
+			return fmt.Errorf("mismatch; actual %#v != expected %#v", actual[i], expected[i])
 		}
 	}
 	return nil