Error handling for bad git data in updatenotification

[shbatm]

The updatenotification module did not ignore errors in the simple-git git.log() call. After some module updates on my installation I started seeing frequent UnhandledPromiseRejectionWarning errors:

1|mm  | 2019-01-04T09:00:09 <error> (node:947) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'hash' of null
1|mm  |     at /home/pi/MagicMirror/modules/default/updatenotification/node_helper.js:68:32
1|mm  |     at Git.<anonymous> (/home/pi/MagicMirror/node_modules/simple-git/src/git.js:1560:10)
1|mm  |     at done (/home/pi/MagicMirror/node_modules/simple-git/src/git.js:1432:21)
1|mm  |     at /home/pi/MagicMirror/node_modules/simple-git/src/git.js:1457:16 (process/warning.js:18 writeOut)
1|mm  | 2019-01-04T09:00:09 <error> (node:947) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1) (process/warning.js:18 writeOut)
1|mm  | 2019-01-04T09:00:09 <error> (node:947) [DEP0018] 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. (process/warning.js:18 writeOut)

This PR aborts the call if data2.latest is undefined for some reason. It is a nuisance error and the only negative side effect should be that we don't know if a module has an update; however, this is the same handling as if the fetch() call failed.

Related to closed issues #1164 and #1285.