`WindowsErrorResult` is a class to hold either a value or a Windows error
code based on the `Result` template. We also have `LauncherResult` for the
same purpose, which was introduced as a part of the launcher process feature
afterward. The difference is `LauncherResult` holds a filename and line
number along with an error code.
This patch integrates LauncherResult.h into WinHeaderOnlyUtils.h so that we
can use `LauncherResult` more broadly.
Differential Revision: https://phabricator.services.mozilla.com/D44512
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extra : moz-landing-system : lando
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