Better error handling and reporting

[tale]

Improve error handling across the Lua runtime and executor:

• Lua errors should include source locations — when a user script errors, the message should clearly show the file, line number, and a readable traceback rather than raw Luau internals.
• rb.exec failure handling — currently commands that exit non-zero are not surfaced well. Add a way to detect and report failed commands (at minimum log the exit code; optionally allow the user to mark commands as allow_fail).
• Validation at plan time — catch common mistakes early (e.g. rb.link_file with a source that does not exist, rb.file with a nil content argument) with clear error messages.
• Executor error recovery — if one operation fails during apply, report it clearly and continue with remaining operations rather than hard-crashing.