common: print out git status in 'make source'

Makefile

@@ -93,7 +93,7 @@ cstyle:
 
 source:
 	$(if $(shell git rev-parse 2>&1), $(error Not a git repository))
-	$(if $(shell git status --porcelain), $(error Working directory is dirty))
+	$(if $(shell git status --porcelain), $(error Working directory is dirty: $(shell git status --porcelain)))
 	$(if $(DESTDIR), , $(error Please provide DESTDIR variable))
 	mkdir -p $(DESTDIR)/nvml
 	echo -n $(SRCVERSION) > $(DESTDIR)/nvml/.version