Fix conditional definition of `b:editorconfig_applied` for external cores

[cxw42]

After f305bc7 (#142), we have:

       if s:UseConfigFiles_VimCore() == 0
            let b:editorconfig_applied = 1
        endif
    elseif s:editorconfig_core_mode ==? 'external_command'
        call s:UseConfigFiles_ExternalCommand()
        let b:editorconfig_applied = 1

I believe that should be

       if s:UseConfigFiles_VimCore() == 0
            let b:editorconfig_applied = 1
        endif
    elseif s:editorconfig_core_mode ==? 'external_command'
        if s:UseConfigFiles_ExternalCommand() == 0   " <====
          let b:editorconfig_applied = 1
        endif   " <====

plus changes to s:UseConfigFiles_ExternalCommand() to return nonzero on shell-exit != 0.

CC @dbarnett

[dbarnett]

Thanks for the heads up! Looks reasonable.