[tfjs-core] do not hang on invalid browser files

[tharvik]

tf.io.browserFiles doesn't fail when loading invalid files. or rather, it fails, but never rejects the promise, making the IOHandler.load hang forever.

wrapping the JSON.parse call in a try/catch and rejecting accordingly did the trick. and a small test to try it out.

note: FileReader.readAsText() is a callback way to go around reading files. the promise-based Blob.text() would make BrowserFile.load simpler and safer (but felt ouf-of-scope here).