core: Only catch errors from `onBeforeUpload`
core: Only catch errors from `onBeforeUpload`

Previously this code would treat any error at all as an `onBeforeUpload`
error, and show the informer. We also lost the stack trace for errors
because `err` is stringified in the handler. This is correct for
`onBeforeUpload` rejections, since they are string reasons, but not for
other errors.

Now only actual `onBeforeUpload` errors are handled that way, and fatal
upload errors end up in the StatusBar like before and in the console,
with a stack trace.

transloadit/uppy,transloadit/uppy,transloadit/uppy,transloadit/uppy