Wrap errors with %w when returning them

Many places where we return errors don't use the %w wrapping, because
they predate that functionality.

This patch changes those to use %w, so callers can inspect the
underlying errors with errors.Is and errors.As if needed in the future.

The text of the errors does not change, and no existing callers inspect
these errors, so there are no behaviour changes.

internal/config/config.go

@@ -44,13 +44,13 @@ func Load(path, overrides string) (*Config, error) {
 	// Load from the path.
 	buf, err := os.ReadFile(path)
 	if err != nil {
-		return nil, fmt.Errorf("failed to read config at %q: %v", path, err)
+		return nil, fmt.Errorf("failed to read config at %q: %w", path, err)
 	}
 
 	fromFile := &Config{}
 	err = prototext.Unmarshal(buf, fromFile)
 	if err != nil {
-		return nil, fmt.Errorf("parsing config: %v", err)
+		return nil, fmt.Errorf("parsing config: %w", err)
 	}
 	override(c, fromFile)
 
@@ -58,7 +58,7 @@ func Load(path, overrides string) (*Config, error) {
 	fromOverrides := &Config{}
 	err = prototext.Unmarshal([]byte(overrides), fromOverrides)
 	if err != nil {
-		return nil, fmt.Errorf("parsing override: %v", err)
+		return nil, fmt.Errorf("parsing override: %w", err)
 	}
 	override(c, fromOverrides)
 
@@ -67,14 +67,14 @@ func Load(path, overrides string) (*Config, error) {
 	if c.Hostname == "" {
 		c.Hostname, err = os.Hostname()
 		if err != nil {
-			return nil, fmt.Errorf("could not get hostname: %v", err)
+			return nil, fmt.Errorf("could not get hostname: %w", err)
 		}
 	}
 
 	// Validate the GiveUpSendAfter value.
 	if _, err := time.ParseDuration(c.GiveUpSendAfter); err != nil {
 		return nil, fmt.Errorf(
-			"invalid give_up_send_after value %q: %v", c.GiveUpSendAfter, err)
+			"invalid give_up_send_after value %q: %w", c.GiveUpSendAfter, err)
 	}
 
 	return c, nil