Error handling for failed API uploads

When you try to upload an image through the wp.com API and it's too large (larger than the site's `max_upload_filesize`), it will fail with an error that doesn't seem relevant:

> The corresponding HMAC for this file does not match

The error is coming from [here](https://github.com/Automattic/jetpack/blob/42b43280d6e316719d90a534b6eb8e63540ba091/class.jetpack.php#L3497) and it's caused because it doesn't check if the uploaded file failed. Inspecting `$file` on a failed upload shows this:

```
array (
'name' => 'img_0008.jpg',
'type' => '',
'tmp_name' => '',
'error' => 1,
'size' => 0,
)
```

Start at class.jetpack.php around line 3497 and inspect how failed uploads are handled before the HMAC check. Reproduce an oversized image upload through the wp.com API using the issue's `$file` values, then verify that the resulting error reflects the upload failure rather than reporting an HMAC mismatch.

Written by the indexing model from the issue text.