These changes introduce a new field, `Raw`, to `diag.Message`. This
field indicates that the contents of the message are not a format string
and should not be rendered via `Sprintf` during stringification.
The plugin std{out,err} readers have been updated to use raw messages,
and the event reader in `pulumi` has been fixed s.t. it does not format
event payloads before display.
Fixes#551.
234f0816e5