Show a warning if the background image can't be loaded

Hi,

The backgroundImage parameter in settings.json has a somewhat permissive path, for example all the following examples work:

• Windows directory separators: C:\\<username>\\Pictures\\some image.png
• Unix directory separators: C:/<username>/Pictures/some image.png
• A mix of both Windows/Unix directory separators: C:\\<username>\\Pictures/some image.png
• URI using file scheme for filesystem access: file://C:/<username>/Pictures/some image.png
• HTTP URI: https://somesite.xyz/image.jpg

Other related issue:

• when backgroundImage points to a non existing file/URI, Windows terminal does not show a warning or error, it could be handled this way:
• on a non existing file on the filesystem: throw an warning/error that the file for the given file path does not exist
• HTTP URI: if the resource returns a status code other than 200 throw an warning/error that the URI is incorrect
• if the file format is unsupported/unreadable: throw an appropriate error

Since users can both use Unix/Windows separators I thought It was the best to describe this issue before filing a documentation feedback.

Configuration:

• Windows version: 10 - 1909 (18363.836) - 64 bits
• Distribution method: compiled for master branch/deployed manually using Visual Studio 2019 (16.5.4)
• Compilation settings: Release/64 bits / all submoduled needed to be checked out in order for compilation to work
• Project compiled: CascadiaCode
• Windows terminal version: - Slightly ahead of v1.0.1401.0 (first public release) pointing to commit: 8265d941b7fbea8b702d3381fa93a92417dec2ce

Terminal configuration:

• settings.json location: C:\Users\<username>\AppData\Local\Packages\WindowsTerminalDev_8wekyb3d8bbwe\LocalState\settings.json