Add warning if esc not installed (jaegertracing#2964)

* Add warning if esc not installed

Signed-off-by: albertteoh <albert.teoh@logz.io>

* Fail the make target

Signed-off-by: albertteoh <albert.teoh@logz.io>

* Move esc command outside conditional block

Signed-off-by: albertteoh <albert.teoh@logz.io>

Makefile

@@ -230,7 +230,14 @@ jaeger-ui/packages/jaeger-ui/build/index.html:
 	cd jaeger-ui && yarn install --frozen-lockfile && cd packages/jaeger-ui && yarn build
 
 cmd/query/app/ui/actual/gen_assets.go: jaeger-ui/packages/jaeger-ui/build/index.html
-	esc -pkg assets -o cmd/query/app/ui/actual/gen_assets.go -prefix jaeger-ui/packages/jaeger-ui/build jaeger-ui/packages/jaeger-ui/build
+	@if ! command -v esc > /dev/null 2>&1 ; then \
+		echo "esc: Command not found" ; \
+		echo "Check:" ; \
+		echo "- esc is installed: 'make install-tools'" ; \
+		echo "- add \$$GOPATH into \$$PATH: 'export PATH=\$$PATH:\$$(go env GOPATH)/bin'" ; \
+		false ; \
+	fi
+	esc -pkg assets -o cmd/query/app/ui/actual/gen_assets.go -prefix jaeger-ui/packages/jaeger-ui/build jaeger-ui/packages/jaeger-ui/build ; \
 
 .PHONY: build-all-in-one-linux
 build-all-in-one-linux: