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Look at publishing instead of exporting to html which works with missing links.
Apparently it doesn't.
I'm still having the same problem of throwing a fatal error --- even when publishing, not exporting --- when encountering a missing (unresolvable) link. This was after doing a clean install of org-mode 8.3.2 (20151005, using the emacs package manager) on top of a clean install of the latest stable version of emacs (24.5) from EmacsForMacOSX. (I'm running the latest version of Mac OS X Yosemite; haven't gotten around to El Capitan.) I then set up publishing the way Rainer suggested in his email. Well, publishing likewise throws an error when it encounters a missing link target, just as does exporting.
> You can use org-lint for this.
I don't mean to be a nag, but I genuinely don't understand why org-mode's former way of dealing with unresolvable links during export was disabled. The old way, namely just marking the problem link in the output file and continuing with the export, made it very easy to search for the problem in the output file. That approach was simple and worked quite well. It also allowed exporting a single .org file, instead of jumping through the hoops of publishing a project. The new way seems like a giant step backwards; it's likely to be a significant barrier to entry for non-expert users.
Can the old way of dealing with unresolvable links be restored, at least as an option?