Use internal/detrand in the construction of our error messages.
This alters whether there is one or two spaces following the "proto:" prefix.
While it is easy for users to still work around this mutation,
sit at least forces them to write test infrastructure to more fuzzily
match on error strings.
Change-Id: I4ddca717526ee3fc4dbb1e0b36cfca8c6e0df36d
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/194038
Reviewed-by: Herbie Ong <herbie@google.com>
09217f08d2