Show error when unsupported video type is selected.

[timmyc]

This branch seeks to inform users that certain video file types are not supported via the renderPreview() in the widget itself. The .mov issue detailed in #171 is a bit complex - and until uploads can be properly restricted ( #128 ) - users will still be able to upload a .mov file which will not function properly when the widget is rendered on the front-end of the site.

So this branch is implementing @jasmussen 's idea - to not render a functioning <video /> preview in the widget for a file type that we know will not work. I borrowed @obenland's error message suggestion:

To Test

• Create a video widget using a supported file type ( .webm, .m4v etc ) verify that the preview renders as expected
• Create a video widget using a .mov file, and verify the preview renders with an error message

[melchoyce]

Looks good 👍

Can we tighten up the padding inside the notice code blocks to padding: 0 3px 0 0px;?

Should "movie" be "video?"

[timmyc]

Thanks for testing it out @melchoyce - addressed the verbiage and padding in 23eaa1b

[jasmussen]

💖