Bug: temp files creation errors are not checked when truncating WAL

[vadiminshakov]

https://github.com/tidwall/wal/blob/master/wal.go#L731
https://github.com/tidwall/wal/blob/master/wal.go#L837

We need to check the error here and clearly return an error message, rather than waiting for os.Rename to give an irrelevant error because there's nothing to rename.