Improve WebDAV error message

[laurent22]

https://discourse.joplinapp.org/t/typeerror-cannot-use-in-operator-to-search-for-d-multistatus-in-null/5759

Detect error condition and throw "Not a valid WebDAV response".

2020-02-10 15:18:07: "TypeError: Cannot use 'in' operator to search for 'd:multistatus' in null
TypeError: Cannot use 'in' operator to search for 'd:multistatus' in null
at WebDavApi.valueFromJson (C:\Users\Poto\AppData\Local\Temp\1WrQk9srrGlegN3qqW05gYgHbd3\resources\app.asar\lib\WebDavApi.js:142:72)
at WebDavApi.arrayFromJson (C:\Users\Poto\AppData\Local\Temp\1WrQk9srrGlegN3qqW05gYgHbd3\resources\app.asar\lib\WebDavApi.js:180:15)
at WebDavApi.handleNginxHack_ (C:\Users\Poto\AppData\Local\Temp\1WrQk9srrGlegN3qqW05gYgHbd3\resources\app.asar\lib\WebDavApi.js:314:30)
at WebDavApi.exec (C:\Users\Poto\AppData\Local\Temp\1WrQk9srrGlegN3qqW05gYgHbd3\resources\app.asar\lib\WebDavApi.js:436:8)
at processTicksAndRejections (internal/process/task_queues.js:85:5)
at async FileApiDriverWebDav.list (C:\Users\Poto\AppData\Local\Temp\1WrQk9srrGlegN3qqW05gYgHbd3\resources\app.asar\lib\file-api-driver-webdav.js:136:18)
at async tryAndRepeat (C:\Users\Poto\AppData\Local\Temp\1WrQk9srrGlegN3qqW05gYgHbd3\resources\app.asar\lib\file-api.js:37:19)
at async FileApi.list (C:\Users\Poto\AppData\Local\Temp\1WrQk9srrGlegN3qqW05gYgHbd3\resources\app.asar\lib\file-api.js:134:18)
at async Synchronizer.lockFiles_ (C:\Users\Poto\AppData\Local\Temp\1WrQk9srrGlegN3qqW05gYgHbd3\resources\app.asar\lib\synchronizer.js:225:18)
at async Synchronizer.checkLock_ (C:\Users\Poto\AppData\Local\Temp\1WrQk9srrGlegN3qqW05gYgHbd3\resources\app.asar\lib\synchronizer.js:241:21)"