NSIS warnings as errors

[worldjoe]

• Version: 6.7.6

• Target: nsis

I use nsis File's /nonfatal flag to allow my developer builds to ignore that large msi's included in our installer aren't in github. The signing server pulls those large msi files into the build directory before running electron builder. The nonfatal flag spits out a warning (by design) instead of an error when the file doesn't exist.

Unfortunately electron-builder treats all warnings as errors. The -WX option is passed to makensis
https://github.com/electron-userland/electron-builder/blob/master/src/targets/nsis.ts#L188

I suggest a flag that can be passed to nsis to optionally disable the -WX flag.

Pull request coming.

[worldjoe]

#764