[Layout restoration] missing file causes restoration to fail

[marcdumais-work]

If between two Theia sessions, a file that was initially opened in an editor disappears, the restoration fails completely when Theia is opened again.

It would probably be better that the layout restoration was "best effort" and continued even if a part of it fails.

see #560 for one example where this can happen. But it could probably also happen if a file was unavailable for some other reason (e.g. switched git branch).

localhost/:1 Uncaught (in promise) A resource provider for 'file:///tmp/a/theia/examples/browser/aaa.txt' is not registered.
Promise rejected (async)
(anonymous) @ shell-layout-restorer.ts:78
ShellLayoutRestorer.inflate @ shell-layout-restorer.ts:73
(anonymous) @ shell-layout-restorer.ts:28
Promise resolved (async)
ShellLayoutRestorer.onStart @ shell-layout-restorer.ts:25
FrontendApplication.startContributions @ frontend-application.ts:110
FrontendApplication.start @ frontend-application.ts:67
start @ index.js:29
Promise resolved (async)
Object.defineProperty.value @ index.js:48
__webpack_require__ @ bootstrap a28e50b24baf5c30315c:688
Object.defineProperty.value @ bootstrap a28e50b24baf5c30315c:789
(anonymous) @ bootstrap a28e50b24baf5c30315c:789