DM: distinguish between reading and parsing in error messages

Change-Id: Ia79101dfc874f767d72014666943724e05ee8483
Reviewed-on: https://skia-review.googlesource.com/c/183803
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>