Improve output of check-formatting in Makefile

The list of files that would change when reformatted is now printed.
Also, parsing errors now make the check fail.

Author: owen-mc

go/Makefile

@@ -32,7 +32,12 @@ autoformat:
 	find . -path '**/vendor' -prune -or -type f -iname '*.go' ! -empty -print0 | xargs -0 grep -L "//\s*autoformat-ignore" | xargs gofmt -w
 
 check-formatting:
-	test -z "$$(find . -path '**/vendor' -prune -or -type f -iname '*.go' ! -empty -print0 | xargs -0 grep -L "//\s*autoformat-ignore" | xargs gofmt -l)"
+	@output=$$(find . -path '**/vendor' -prune -or -type f -iname '*.go' ! -empty -print0 | xargs -0 grep -L "//\s*autoformat-ignore" | xargs gofmt -l 2>&1); \
+	if [ -n "$$output" ]; then \
+		echo "The following files need to be reformatted using gofmt:"; \
+		echo $$output; \
+	fi; \
+	test -z "$$output"
 
 ifeq ($(QHELP_OUT_DIR),)
 # If not otherwise specified, compile qhelp to markdown in place