Update FileUpload.vue, fix for "_vm.getFileErrors(...).join is not a function" error

[itcyborg]

Fix for issue: getcandy/hub#53
Check if an array of errors is returned or a String.

[alecritson]

Hi @itcyborg

Thanks for this PR, it would be good to some more information about the error you got, just so we can be sure adjusting the hub is the correct way to go and it's not something that needs changing on the API side.

So just a couple of questions:

1. What steps were taken to reproduce this?
2. Was the error from the API or generated client side?
3. What was the actual error that got displayed when this was changed?

[itcyborg]

Steps to reproduce

1. Install the published version of hub and using the boilerplate for candy-api.
2. Add a product using hub
3. Try to upload an image using the provided form in hub. (I tried using bmp, jpg and png; only one image uploaded at a time)
4. Open devtools console then upload, an error "_vm.getFileErrors(...).join is not a function" will be thrown in the console.
5. This error is specifically on the frontend, as it is thrown before the request is sent to the api. I'm still auditing and bug-checking the api, thought I'll be raising issues and possible PR for them soon. The above error indicates that it is only assuming that errors will return an array and not a string.

• I did the update manually on my side though I can't build without moving the edited node modules manually without repackaging already made packages. It worked for my case but I've managed to reproduce it on my dev and staging servers using direct builds from the mainstream.

In case of further queries, let me know.