Restore unreachable error handling when plugin::byId can't find a plugin

[Salvialf]

• plugin::byId() is the only byId() method in the entire core/class/ directory that throws an exception instead of returning a falsy value when nothing is found (verified against all 27 other byId() implementations, which are plain DB lookups). Several call sites across core were written assuming the more common convention, guarding the result with if (!is_object($plugin)). Since byId() throws before ever returning, these guards were dead code, and the intended fallback behavior (a friendly error message, a graceful "not found" response, skipping one entry in a loop) was never reachable.
• Each affected call now wraps plugin::byId() in a try/catch (or, in utils.inc.php's loop over multiple plugins, a plugin::isInstalled() pre-check), restoring the originally intended fallback behavior instead of letting a generic exception propagate to the nearest outer handler.
• In core/ajax/plugin.ajax.php, also removed two $plugin_id = init('id') assignments left unused in getDeamonInfo and deamonStart, found while touching these same blocks.
• jsonrpc::makeError() and jsonrpc::makeSuccess() are now documented with @return never, mirroring ajax::success()/ajax::error()'s existing annotations. Without it, PHPStan flagged $plugin as possibly undefined after the try/catch blocks in core/api/jeeApi.php, since it couldn't infer that the catch branch's call to makeSuccess() always ends execution.