phosphor-software-manager: verify vendor bundle existence before access This avoids error-level logs in cases where the vendor bundle version file is not present on a machine. Fusion-Link: fusion2 N/A Tested: https://paste.googleplex.com/4545763326689280 Google-Bug-Id: 490359116 Change-Id: I6ec6f56c38299a067f05f4ad216e8ce3e566c8ef Signed-off-by: Pearl Hwang <pearlhwang@google.com> (cherry picked from commit 114b28be6c16c58e62aa53440d7a215f4d1062ea) Platforms-Affected: GB300