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)
 			}
 		}
 	}