fix 5742: It should pop up Error opening file dialog if click the image which is deleted from OS.
Merge branch 'master' into couzteau/fix-5742-Error-on-deleted-img

Conflicts:
	src/editor/ImageViewer.js
review feedback - removed the need for  ImageViewer or other CustomViewers to display a warning when an image is renamed or deleted while viewed.
remove unused modules from require section
remove unused var
Do not show error when file is deleted or renamed, instead just close image and open alternate file. Made this change to stay consistent with how text files behave in the same situation as well as to avoid a clunky error message.
Merge branch 'master' into couzteau/fix-5742-Error-on-deleted-img
update to new file.exists semantics
add missing argument, thanks @RaymondLim
fix issue where image was removed when brackets gained focus. Part 2 of update to integrate change file.exists API
another minor style change
- Show generic error or File-Not_Found when failing to open an image file.
- Added JSDoc
Merge pull request #5910 from adobe/couzteau/fix-5742-Error-on-deleted-img

fix 5742: It should pop up Error opening file dialog if click the image which is deleted from OS.