Show an error when FUCHSIA_DIR is not defined.

TEST: Manually ran `make run` without FUCHSIA_DIR
diff --git a/Makefile b/Makefile
index c0465ac..d681765 100644
--- a/Makefile
+++ b/Makefile
@@ -53,6 +53,9 @@
 
 run: $(if $(wildcard frontend/dist),,frontend)
 run: $(if $(wildcard $(SERVER)),,backend)
+ifndef FUCHSIA_DIR
+	$(error FUCHSIA_DIR is not defined. Either pass in FUCHSIA_DIR="path" or define the variable.)
+endif
 ifeq ($(COPY),1)
 	./copy_fuchsia_files.sh
 endif