Print out subcommand when it fails (#2585)

--verbose_failures only prints out the command that Bazel rules call directly. If a binary calls several subcommands (e.g., compilepkg), we don't know which subcommand fails until we run the parent command manually. For failures that cannot be reproduced locally (e.g., the Windows errors of #2461), printing out the subcommand gives us better idea what could have gone wrong.