Desktop silently ignores failures when opening the default file manager

Description

Desktop silently ignores failures when opening a project in the default file manager. Electron's shell.openPath() resolves with an error string on failure, but the open-path IPC handler forwards that as a successful result. The existing UI error handler only runs when the promise rejects.

For example, a project directory that has been removed can fail to open without showing an error. Convert a non-empty result into a rejected error so the existing request-error toast can explain the failure.

OpenCode version

Source review on dev at 56d4fb24a1 (1.18.29).

Steps to reproduce

At the file-opening boundary, have the default opener resolve with an error message, as specified by Electron's API. The current handler resolves successfully instead of rejecting. A regression test reproduces this behavior; a full desktop UI reproduction has not been performed.

Manual check: open a disposable project in Desktop, remove its directory externally, then choose Open in File Manager. A failed open should display an error rather than silently completing.

Operating System

Platform-independent IPC handling; boundary tests run on macOS.