Ensure all promises have exception handlers

[DonJayamanne]

This applies to commands and other VS Code API that return promises.

Note: We seem to have code within the extension where this happens today.
See #218 (comment)

UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): TypeError: Cannot read property 'SendExceptionInfo' of null
(node:6204) DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

[DonJayamanne]

Fixed with #228 (changes committed)