handle not found files for plugins properly

[akosyakov]

What it does

handle not found files for plugins properly:

• don't fail if a file is found
• don't swallow an error bug log it
• only send 404 if request was not responded already

How to test

1. Clone https://github.com/vinokurig/Test.git under plugins and compile it
2. Start the browser example
3. Open the node dependency view
4. Check that elements have icons and no errors in logs

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[akosyakov]

@yeweiasia Could you check whether you can reproduce #5967 with this PR?

[tsmaeder]

The only "danger" I see with this change is that there is a code path that does not send a response to the client. The old code would always send a response, even if too late in some cases.

[akosyakov]

The only "danger" I see with this change is that there is a code path that does not send a response to the client. The old code would always send a response, even if too late in some cases.

The old code was always failing because it was trying to send an error response when a file was found or request was already closed with another error.

[akosyakov]

@tsmaeder I've updated code with comments and mapping all other errors to 500.

[tsmaeder]

@akosyakov thx, lgtm.

[vinokurig]

Tested with https://github.com/vinokurig/Test.git plugin. The plugin works as expected, no errors in the console

[akosyakov]

@vinokurig please rebase your PR, i will test it then

[yeweiasia]

@yeweiasia Could you check whether you can reproduce #5967 with this PR?

I synced the code with mater HEAD, It seems that all vscode plugin fail to start properly, I don't have much time to diagnose today, deep diagnostics can be provided this weekend.