Better experience for non-matching reivewiers/labels

[miguelsolorio]

Issue Type: Bug

Testing https://github.com/Microsoft/vscode/issues/69254

Whenever I enter a label or reviewer that doesn't exist, there is nothing communicated about what has happened (did it add it? was it an error? what happened?). We should provide a better experience by either showing validation or a notification that those users/labels don't exist.

Repro steps

1. Open a PR, click on add label
2. Type in a name of a label that doesn't exist
3. Click "Ok"
4. 🐛 No information is communicated on what has happened

Extension version: 1.56.0 VS Code version: Code - Insiders 1.32.0-insider (393b48d4a5de85b6fa1d2ad0a59588d6715e12ea, 2019-02-25T01:33:51.804Z) OS version: Darwin x64 18.2.0

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-7920HQ CPU @ 3.10GHz (8 x 3100)
GPU Status	2d_canvas: enabled checker_imaging: disabled_off flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on native_gpu_memory_buffers: enabled rasterization: enabled surface_synchronization: enabled_on video_decode: enabled webgl: enabled webgl2: enabled
Load (avg)	2, 2, 2
Memory (System)	16.00GB (0.63GB free)
Process Argv	-psn_0_569483
Screen Reader	no
VM	0%

[alexr00]

The best way to do this would be with validation. Upstream issue: https://github.com/microsoft/vscode/issues/187863