Rollup merge of #34312 - erickt:add-try, r=nikomatsakis

Revert using ? for try! in the libsyntax pretty printer

The use of ...?instead of try!(...) in libsyntax makes extracting libsyntax into syntex quite painful since it's not stable yet. This makes backports take a much longer time and causes a lot of problems for the syntex dependencies. Even if it was, it'd take a few release cycles until syntex would be able to use it. Since it's not stable and that this feature is just syntax sugar, it would be most helpful if we could remove it.

cc #34311

Author: Manishearth