Add wal validation debug information on error

This commit adds the WAL header and shadow path to "wal header mismatch"
errors to help debug issues. The mismatch seems to happen more often
than I would expect on restart. This error doesn't cause any corruption;
it simply causes a generation to restart which requires a snapshot.

db.go

@@ -462,7 +462,7 @@ func (db *DB) init() (err error) {
 
 	// If we have an existing shadow WAL, ensure the headers match.
 	if err := db.verifyHeadersMatch(); err != nil {
-		log.Printf("%s: init: cannot determine last wal position, clearing generation (%s)", db.path, err)
+		log.Printf("%s: init: cannot determine last wal position, clearing generation; %s", db.path, err)
 		if err := os.Remove(db.GenerationNamePath()); err != nil && !os.IsNotExist(err) {
 			return fmt.Errorf("remove generation name: %w", err)
 		}
@@ -512,7 +512,7 @@ func (db *DB) verifyHeadersMatch() error {
 	}
 
 	if !bytes.Equal(hdr0, hdr1) {
-		return fmt.Errorf("wal header mismatch")
+		return fmt.Errorf("wal header mismatch %x <> %x on %s", hdr0, hdr1, shadowWALPath)
 	}
 	return nil
 }