Valid entries returned by `fetchFileAtUri` in UssFSProvider for files that do not exist

[t1m0thyj]

Similar to #3252 which was for Data Sets, but this issue is for the USS tree - if I attempt to open a file and the getContents() call throws an error for some reason, then a blank file is opened in the editor and no error message is displayed.

Steps to Reproduce
An easy way to reproduce this without using an extender is to modify the implementation of ZoweExplorerZosmf.UssApi.getContents to throw an error.

Actual Behavior
If I attempt to open a file and the getContents() call throws an error for some reason, then a blank file is opened in the editor and no error message is displayed.

Expected Behavior
I would expect an error to be shown to match the behavior for data sets:

[github-actions]

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