Removed unused upload.html
Removed unused google-youtube.html import.
Changed the uploadFiled method to take in a file parameter, in case
someone wants to call the method directly without using the controls in
the element. Also, disabled the Upload button unless there’s a file
selected.
Present meaningful error text even when the cause of the failed
upload/processing error is unknown.
Added JSDoc for the uploadFile method, which might as well be publicly
exposed.
Modified the inline example in the JSDocs to show off some additional
attributes.
More JSDoc updates.
Added some more explanation to the clientId JSDoc, since registering
can be confusing.