Improve NDK build error handling

Change-Id: I1dd51d8c5d3b8eef829f60a72c8c869e18b64830

jni/Android.mk

diff --git a/jni/Android.mk b/jni/Android.mk
index 4ddcdf8..f959ea8 100644
--- a/jni/Android.mk
+++ b/jni/Android.mk
@@ -79,6 +79,11 @@
   $(error Ceres requires NDK version r9d or greater)
 endif
 
+# Ceres requires Eigen to build.
+ifndef EIGEN_PATH
+  $(error Ceres requires Eigen; please invoke via EIGEN_PATH=... ndk-build)
+endif
+
 EIGEN_PATH := $(EIGEN_PATH)
 CERES_INCLUDE_PATHS := $(CERES_EXTRA_INCLUDES)
 CERES_INCLUDE_PATHS += $(LOCAL_PATH)/../internal