Handle invalid book id tags more robustly

[Enkidu93]

We've seen quite a few errors thrown recently in Serval regarding invalid book ids. Many of these involve extra characters after the correct book id. Also, many involve having more than one id tag before the first chapter of a book. We should:

1. Only grab the first three characters of a book id code (so '\id MATother-text' will be treated as '\id MAT').
2. And we should ignore \id tags after the first. We already do this in the UsfmParser; we just don't do it when performing error checking in StartBook in the handler.