Make `-Z treat-err-as-bug` accept a number of errors to skip

[estebank]

Allow -Z treat-err-as-bug to take a number, in order to make it easier to debug errors that can't be made to be emitted first. In my mind, you'd call rustc -Z treat-err-as-bug 2 on a file with three errors and the first two would be emitted, you'd get the backtrace for the second one, and the third one wouldn't be emitted.