Better exception handling when parsing responses received from Jedi

[DonJayamanne]

See #1721 (comment) & #1810 (comment)

ERR Cannot read property 'id' of undefined: TypeError: Cannot read property 'id' of undefined at responses.forEach (C:\Users\derekw\.vscode-insiders\extensions\ms-python.python-2018.4.0\out\client\providers\jediProxy.js:393:54) at Array.forEach (native)