Finish renaming BUILD_DIR; make tests more error-prone

I had forgotten that I needed to change the value of BUILD_DIR
in several different places.

Added comments warning future editors about this as well as filing
issue #5007 to track fixing this properly.

Despite being misconfigured and therefore failing, the typescript
and metadata test scripts were exiting with status 0, indicating
successful completion.  These have been fixed so they should fail
on any error, including misconfiguration.

scripts/gulpfiles/config.js

@@ -12,6 +12,12 @@ var path = require('path');
 
 // Paths are all relative to the repository root.  Do not include
 // trailing slash.
+//
+// TODO(#5007): If you modify these values, you must also modify the
+// corresponding values in the following files:
+// 
+// - tests/scripts/compile_typings.sh
+// - tests/scripts/check_metadata.sh
 module.exports = {
   // Directory to write compiled output to.
   BUILD_DIR: 'build',