tweak build_sdk to display color unified diffs, and move diff to after the build succeeds
tweak build_sdk to display color unified diffs, and move diff to after the build succeeds

when debugging js_codegen errors, this makes it a lot easier to see the stack trace.

R=vsm@google.com

Review URL: https://codereview.chromium.org/1153313005

ochafik/dev_compiler,ronlobo/dev_compiler,ronlobo/dev_compiler,ochafik/dev_compiler,ochafik/dev_compiler,ochafik/dev_compiler,ronlobo/dev_compiler,ronlobo/dev_compiler