dirstate-v2: Add a zero-size error type for dirstate v2 parse errors This error should only happen if Mercurial is buggy or the file is corrupted. It indicates for example that: * A part of the file refers to another part, and the byte offset or item count would cause reading out of bounds, beyond the end of the file. * The byte for an entry state has an invalid value When parsing becomes lazy, many more functions will return a `Result` with this error. Making it zero-size reduces the work that the `?` operator needs to do to pass around the error value. Differential Revision: https://phab.mercurial-scm.org/D10748