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.

lib/config/config.go

@@ -295,11 +295,11 @@ func (cfg *Configuration) clean() error {
 		}
 
 		if folder.Path == "" {
-			return fmt.Errorf("folder %q: %v", folder.ID, errFolderPathEmpty)
+			return fmt.Errorf("folder %q: %w", folder.ID, errFolderPathEmpty)
 		}
 
 		if _, ok := existingFolders[folder.ID]; ok {
-			return fmt.Errorf("folder %q: %v", folder.ID, errFolderIDDuplicate)
+			return fmt.Errorf("folder %q: %w", folder.ID, errFolderIDDuplicate)
 		}
 
 		existingFolders[folder.ID] = folder