The tab completion handler in `completePrefix` only caught `ParseError`,
`EvalError`, `BadURL`, and `FileNotFound`. Other error types like
`JSONParseError` (which derives from `Error`, not `EvalError`) escaped
the catch block and propagated through editline's C code as undefined
behavior, crashing the REPL. This happened when tab-completing
expressions like `(builtins.fromJSON "invalid").` where evaluation
throws a non-`EvalError` exception.
This commit marks `completionCallback` and `listPossibleCallback` as
`noexcept` with function-try-blocks that catch all exceptions at the
C/C++ boundary, preventing any exception from reaching editline.
Fixes#15133.
