Merge branch 'errors-phase-2' of https://github.com/bburdette/nix

src/build-remote/build-remote.cc

src/error-demo/error-demo.cc

src/error-demo/local.mk

src/libexpr/attr-path.cc

src/libexpr/attr-set.hh

src/libexpr/eval-inline.hh

src/libexpr/eval.cc

src/libexpr/lexer.l

src/libexpr/nixexpr.cc

src/libexpr/nixexpr.hh

src/libexpr/parser.y

src/libexpr/primops.cc

src/libexpr/primops/context.cc

src/libexpr/primops/fetchGit.cc

src/libexpr/primops/fetchMercurial.cc

src/libexpr/primops/fetchTree.cc

src/libexpr/primops/fromTOML.cc

src/libexpr/value-to-json.cc

src/libmain/progress-bar.cc

src/libmain/shared.cc