Prefer returning bool/enum values over integers

[jhauberg]

In the most basic cases, where a function is indeed only either successful or not, bool should always be used.

In cases where a non-successful result carries more information, an enum should be used. For example, lxt_parse, which can hit several different failures, should return which exact failure it hit (e.g. max containers, entries etc.).

It might even be worthwhile to go further than just simple enums and instead return stateful information so that the library can report exactly where and why something went wrong; e.g. parsing errors at line and column numbers. But that is probably better fit as a separate issue.