build: improve UX when external dep build fails under Bazel. (#1591)
build: improve UX when external dep build fails under Bazel. (#1591)

The underlying issue is we aren't hard failing the build when a
repository_rule fails, which Bazel should presumably be doing, see
https://github.com/bazelbuild/bazel/issues/3683.

The PR prints a brightly colored warning that provides guidance on where
to look to resolve the external dep failure. See
https://user-images.githubusercontent.com/10914751/30034498-f04e2dea-916f-11e7-9ab4-000c37f62307.png.

Fixes #1589.
jrajahalme/envoy,dnoe/envoy,istio/envoy,envoyproxy/envoy,fengli79/envoy,lyft/envoy,rlazarus/envoy,envoyproxy/envoy,dnoe/envoy,lizan/envoy,tdmackey/envoy,envoyproxy/envoy-wasm,amalgam8/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,jrajahalme/envoy,dnoe/envoy,lizan/envoy,envoyproxy/envoy,fengli79/envoy,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy,tdmackey/envoy,envoyproxy/envoy-wasm,lyft/envoy,envoyproxy/envoy,lyft/envoy,jrajahalme/envoy,istio/envoy,eklitzke/envoy,lyft/envoy,lyft/envoy,tdmackey/envoy,amalgam8/envoy,dnoe/envoy,rlazarus/envoy,envoyproxy/envoy-wasm,tdmackey/envoy,craffert0/envoy,envoyproxy/envoy,istio/envoy,lizan/envoy,lizan/envoy,eklitzke/envoy,envoyproxy/envoy,eklitzke/envoy,jrajahalme/envoy,istio/envoy,jrajahalme/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,sebrandon1/envoy,istio/envoy,rlazarus/envoy,sebrandon1/envoy,envoyproxy/envoy,eklitzke/envoy,istio/envoy,eklitzke/envoy,jrajahalme/envoy,craffert0/envoy,istio/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,fengli79/envoy,rlazarus/envoy,eklitzke/envoy,fengli79/envoy,lyft/envoy,envoyproxy/envoy,lizan/envoy,craffert0/envoy,craffert0/envoy,dnoe/envoy,amalgam8/envoy,sebrandon1/envoy,sebrandon1/envoy,amalgam8/envoy,sebrandon1/envoy