Improve the grammar error handling

[marek-hradil]

Grammar constructors have now pretty large number of exceptions on which they panic. Some of them are even not documented in the doc string of the functions, maybe because even llama.cpp does not mention them.

For example, when the grammar_str does not include grammar_root, upon calling grammar_lazy it just throws

Caused by:
  process didn't exit successfully: `...` (signal: 6, SIGABRT: process abort signal)

which is terrible for figuring out what is wrong (ive been there).

This PR fixes this, adding GrammarError and sanitizing the inputs to grammar and grammar_lazy functions.
The change is however breaking as we need a way to propagate the errors - the output type thus moved from Option<LlamaSampler> to Result<LlamaSampler, GrammarError>.

[MarcusDunn]

the grammar code was created just as llama.cpp started it and has been untouched since. Thanks for taking the time to improve it!