update to protocompile v0.7.1

[jhump]

As of v0.7.0, some incompatibilities were introduced in protocompile, due to changes/improvements to its ErrorWithPos type, to support reporting a region of code for an error instead of just a single position. So this module's protoparse.ErrorWithSourcePos had to be updated to accommodate that change. That version also depends on a commit of google.golang.org/protobuf that has a newer version of google/protobuf/descriptor.proto, necessitating some updates of golden output files that correspond to descriptor.proto. There have also been tweaks to the parser in protocompile to make the parser able to return a more complete AST, even in the face of some common kinds of errors. This means some syntax error messages have changed, which required updating some test assertions about errors.