`--error-format=short` should be an unstable option

[nrc]

i.e., require -Zunstable-features. This should be backported to beta if necessary.

cc @GuillaumeGomez @alexcrichton @estebank

[Mark-Simulacrum]

Setting priority to medium -- ideally we'd get this done before beta branched, but it definitely needs to land before 1.23 stable releases.

[GuillaumeGomez]

I'll add it very soon.

[nikomatsakis]

triage: P-high (I'd prefer to keep my eye on this)

But looks like @GuillaumeGomez is all over it. Thanks @GuillaumeGomez.