Gracefully handle cancelled 'xml/closeTag' requests

[fbricon]

With eclipse/lemminx#412 enabled, rapidly typing closing tags in a large xml document leads to vscode logging errors:

rejected promise not handled within 1 second: Error: The request (id: 19, method: 'xml/closeTag') has been cancelled
extensionHostProcess.js:749
stack trace: Error: The request (id: 19, method: 'xml/closeTag') has been cancelled
	at handleResponse (/Users/fbricon/Dev/projects/vscode-xml/dist/extension.js:6061:48)
	at processMessageQueue (/Users/fbricon/Dev/projects/vscode-xml/dist/extension.js:5888:17)
	at Immediate.module.exports../node_modules/vscode-jsonrpc/lib/main.js.setImmediate (/Users/fbricon/Dev/projects/vscode-xml/dist/extension.js:5872:13)
	at runCallback (timers.js:696:18)
	at tryOnImmediate (timers.js:667:5)
	at processImmediate (timers.js:649:5)

Cancellations should be handled more gracefully