Fix #4089 Linked account cancel (#5347)

VSCode serialization changed rejection handling to only serialize errors, which caused things to break
- Changed to return either account info or a cancel message in the resolve
- Rewrote to use promises. Tracking how to return canceled through 4+ thenables was way trickier than just using a promise
- Updated unit tests to handle new scenario
- Tested integration tests, realized they a) didn't run and b) didn't passed. 
  - Added vscode dev dependency to fix run issue
  - Fixed tests to account for behavior changes in tree state.

extensions/azurecore/package.json

@@ -6,7 +6,7 @@
 	"publisher": "Microsoft",
 	"preview": true,
 	"engines": {
-		"vscode": "^1.25.0",
+		"vscode": "^1.30.1",
 		"azdata": "*"
 	},
 	"activationEvents": [
@@ -169,6 +169,7 @@
 		"@types/request": "^2.48.1",
 		"mocha": "^5.2.0",
 		"should": "^13.2.1",
-		"typemoq": "^2.1.0"
+		"typemoq": "^2.1.0",
+		"vscode": "1.1.26"
 	}
 }