docs: fix exception-handling examples and Lua.new option doctests

[davydog187]

What

Fixes three classes of documentation mistake around exception handling and doctest rendering. All examples were verified by running them.

1. README: e.message returns nil

The "Error messages with source and line" tour rescued a Lua.RuntimeException from error("...") and read e.message. Since #384, that field is nil when the exception wraps a VM error — the message is rendered lazily via Exception.message/1. Switched the example to the accessor.

The illustrative comment was also wrong: it showed a Runtime Error title line the formatter never emits. Corrected to the actual (ANSI-elided) frame:

Lua runtime error: at <eval>:2:

  runtime error: something went wrong

e.line/e.source were already correct and are unchanged.

2. Lua.new/1 option doctests rendered as plain text

The three ## Options doctests (:max_call_depth, :max_string_bytes, :max_instructions) were indented 6 spaces. Nested under 2-space bullets, a code block needs 8 spaces — so Markdown/ExDoc rendered them as lazy continuation paragraphs instead of highlighted code blocks. Re-indented all three (confirmed via mix docs that each now renders inside a <pre> block within its bullet).

Also replaced the double-escaped \"x\" in the :max_string_bytes example with a single-quoted Lua string ('x') so it renders cleanly.

3. Marketing snippets used a function that doesn't exist

website/.../snippets.ex called Lua.eval/2 (only Lua.eval!/2 exists) in three places, and the sandbox snippet read the nil .message field with the wrong expected substring. Rewrote to idiomatic Lua.eval!/2 (correct {results, lua} shape; the sandbox one now shows the raise, matching os.execute(_) is sandboxed).

Verification

• mix test --only doctest — 49 pass (the three re-indented Lua.new/1 doctests included).
• mix docs — the three option examples now render as code blocks; \"x\" escaping gone.
• mix format --check-formatted lib/lua.ex — clean.
• Website compiles.