cmd/tip: add context to error values and log prints
cmd/tip: add context to error values and log prints

To help track down some flakiness.

Change-Id: I2dd21abdc0f05c58857da580d962b0ab72cf52b3
Reviewed-on: https://go-review.googlesource.com/20728
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build