core: make genesis incompatibility error more explicit

core/genesis.go

@@ -86,7 +86,7 @@ type GenesisMismatchError struct {
 }
 
 func (e *GenesisMismatchError) Error() string {
-	return fmt.Sprintf("wrong genesis block in database (have %x, new %x)", e.Stored[:8], e.New[:8])
+	return fmt.Sprintf("database already contains an incompatible genesis block (have %x, new %x)", e.Stored[:8], e.New[:8])
 }
 
 // SetupGenesisBlock writes or updates the genesis block in db.