improve scripts/build-gui.js error reporting, compatibility

Author: Christopher Willis-Ford

scripts/build-gui.js

@@ -1,15 +1,19 @@
 const childProcess = require('child_process');
 const path = require('path');
 
-childProcess.spawnSync(
+const child = childProcess.spawnSync(
     'npm', ['run', 'build'],
     {
         cwd: path.join('node_modules', 'scratch-gui'),
         env: {
+            ...process.env,
             BUILD_MODE: 'dist',
             STATIC_PATH: 'static'
         },
         shell: true,
         stdio: 'inherit'
     }
 );
+
+if (child.error) throw child.error;
+process.exit(child.status);