dashboard/builder: fix bugs
dashboard/builder: fix bugs

1) Killing proceses on timeout was wrong: the os/exec
package will never return our package's error type
from its Wait.

2) fix a goroutine leak on timeout.

3) unexported an undocumented and elsewhere-unused type.

4) rename timeout type to end in "Error", per convention,
not Err.

LGTM=adg
R=adg
CC=golang-codereviews
https://codereview.appspot.com/74290043

skyportsystems/go.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,skyportsystems/go.tools