Publish to azure in VS2015 with message: The 'prepublish' script failed with status code 1

I took an existing ASP.NET 4/MVC 5 web site and manually converted to an ASP.NET 5/MVC 6 project.

When I got the application fully functional locally I attempted to publish to the previous web app on Azure. To do this I:

1. Downloaded the publish profile from the Azure portal.
2. Right clicked the web project and selected "Publish".
3. Imported the publish profile and attempted to publish.

Amazingly it worked the first time. However on subsequent publish attempts I received the error:

Error : The 'prepublish' script failed with status code 1.

Looking at the output of the publish I see the following:

[13:25:07] 'min:ngAppJs' errored after 307 ms
[13:25:07] Error: EPERM, open 'C:SCMHolos Portal 1.0 (ASP.Net 5, MVC6)HolosPortalsrcHolosPortalwwwrootngAppngApp.min.js' C:Program Files (x86)MSBuildMicrosoftVisualStudiov14.0WebMicrosoft.DNX.Publishing.targets(152,5): Error : The 'prepublish' script failed with status code 1.

2>Publish failed due to build errors. Check the error list for more details.

I eventually discovered that if I delete the output file (ngApp.min.js) the publish would succeed but then fail again on the next publish.

I had created a new min task for my angular app

gulp.task("min:ngAppJs", function () {
    retu gulp.src([paths.ngAppJs, "!" + paths.ngAppMinJs], { base: "." })
        .pipe(concat(paths.concatNgAppJsDest))
        .pipe(uglify())
        .pipe(gulp.dest("."));
});

but I had neglected to create a cleanup task to remove the output file

gulp.task("clean:ngAppJs", function (cb) {
    rimraf(paths.concatNgAppJsDest, cb);
});

gulp.task("clean", ["clean:js", "clean:ngAppJs", "clean:css"]);

Adding the clean task to gulpfile.js solved the problem.