Makefile: add verify-clean and verify-generate targets

Use `make verify` to check if someone forgot to run `go generate`
after making changes in errors.go.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Makefile

@@ -5,7 +5,7 @@ all:
 	CGO_ENABLED=0 $(GO) build -ldflags="-X main.Commit=$$(git describe --tags --abbrev=8 --dirty --always --long)"
 
 .PHONY: verify
-verify: verify-space verify-golangci
+verify: verify-space verify-generate verify-golangci
 
 .PHONY: verify-golangci
 verify-golangci:
@@ -17,3 +17,12 @@ verify-space: ## Ensure no whitespace at EOL
 		echo "^^^^ extra whitespace at EOL, please fix"; \
 		exit 1; \
 	fi
+
+.PHONY: verify-clean
+verify-clean:
+	git diff --exit-code ## Were the changes committed?
+
+.PHONY: verify-generate
+verify-generate: verify-clean
+	go generate
+	git diff --exit-code ## Did go generate produced anything new?