sweet: error on unknown fields in config toml

It took me two days to notice that my `buildenv` field was a typo
(should be `envbuild`), and was thus silently ignored.

Make this an immediate error instead.

Change-Id: Iea72d8874404c9ccc6f6c7518417fb06d4e7c1bf
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/446015
Auto-Submit: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
1 file changed