keymaps: 'Open Keyboard Shortcuts in JSON' command should create file if it does not exist

[vince-fugnitto]

Bug Description:

The command Open Keyboard Shortcuts in JSON should create the keymaps.json file if it does not exist instead of throwing an error:

monaco-editor-provider.js:203 Uncaught (in promise) Error: 'user_storage:/keymaps.json' is invalid
    at MonacoEditorProvider.<anonymous> (monaco-editor-provider.js:203)
    at step (monaco-editor-provider.js:71)
    at Object.next (monaco-editor-provider.js:52)
    at fulfilled (monaco-editor-provider.js:43)

Steps to Reproduce:

1. make sure that the keymaps.json file does not exist (ex: rm -rf ~/.theia)
2. open the keyboard shortcuts view, and attempt to open the keymaps.json with the toolbar item ({ })
3. the error is thrown

[akosyakov]

New fs API regression? Could you check how it works before? If it is used to please assign to me.

[vince-fugnitto]

New fs API regression? Could you check how it works before? If it is used to please assign to me.

I also noticed regressions with actually using the widget to write keymaps updates.
I'm rebuilding at the moment to confirm this regression as well.

[vince-fugnitto]

The keymaps.json file is created in v1.4.0, I believe it is a regression.

[akosyakov]

We replaced UserStorageResoure with UserStorageFileSystemProvider. There was a bug in UserStorageResource that it won't throw not found exception if file does not exist and clients don't handle it well. With UserStorageFileSystemProvider exception is thrown properly. We have to adjust clients to respect it.