Bug 13150 – Add ability to log gagged compiler errors to file

DMD does a lot of gagging and although ideally it would not have to do any gagging at all, it's going to be a long time before this happens. One problem with gagging is that it sometimes hides useful error messages (sometimes even your own static assert with a custom message). I propose we add an option to emit all gagged errors to a file. That way we can at least attempt to trace through the compiler stages to see what went wrong. This would require a new switch.

-verrors=spec was introduced in 2.072.0 which prints error messages from speculative compiles.