Cleanup build_sdk error checking

pkg/dev_compiler/tool/build_sdk.sh

 #!/bin/bash
 set -e
 # switch to the root directory of dev_compiler
 cd $( dirname "${BASH_SOURCE[0]}" )/..
 
 echo "*** Patching SDK"
 dart -c tool/patch_sdk.dart tool/input_sdk gen/patched_sdk
 
 echo "*** Compiling SDK to JavaScript"
 
 # TODO(jmesserly): break out dart:html & friends.
 { # Try
   dart -c tool/build_sdk.dart \
     --dart-sdk gen/patched_sdk \
     --dart-sdk-summary=build \
     --summary-out lib/sdk/ddc_sdk.sum \
     --modules=amd \
     -o lib/js/amd/dart_sdk.js \
     --modules=es6 \
     -o lib/js/es6/dart_sdk.js \
     --modules=common \
     -o lib/js/common/dart_sdk.js \
     --modules=legacy \
     -o lib/js/legacy/dart_sdk.js \
     "$@" > tool/sdk_expected_errors.txt
 } || { # Catch
   # Show errors if the sdk didn't compile.
   cat tool/sdk_expected_errors.txt
+  exit 1
 }