It seems like if you have an userscript, which has set the @icon metadata and the file it's pointing at doesn't exist, the userscript can't be installed.
This can be problematic if the server the icon is hosted on is down, because then the userscript can't be installed.
GM should use some "default image" if this kind of problem occurs, there is no point in disallowing a script to install if only some icon is missing.
That might be, but it seems like if the server returned another error code other than 404 it fails.
This is just the standard "I was trying to download the script and got an error" message. It would be possible to ignore errors for the @icon. Even maybe reasonable.
Do not fail to install the script if just the @icon fails to download.