tools/veyron: print warnings to stderr instead of stdout.
tools/veyron: print warnings to stderr instead of stdout.

This makes it easy to treat them separately from the stdout stream.

Change-Id: I4b5a1d7113a61e0ef509cf656721257d717002c9

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools