rustc: Suppress derived pattern-match-checking errors

typeck::check::_match wasn't suppressing derived errors properly.
Fixed it.
rustc: Change At to Managed and Uniq to Owned
rustc / test: Fix error message
Fix error messages harder
auto merge of #6103 : catamorphism/rust/nonfatal-errors, r=catamorphism

r? @nikomatsakis typeck::check::_match wasn't suppressing derived errors properly.
Fixed it. (This will fix #5100)