Better error message when jarsigner isn't found
Better error message when jarsigner isn't found

Reviewed By: emma0303, justinjhendrick

Differential Revision: D13864652

fbshipit-source-id: 8f0f44f4c35594a6d9aaae1fe91b9bc392515d9b

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex