clearer error if project filepath does not exist

Bug: 148102969
Test: add nonexistent path in settings.gradle, observe that the error message mentions the nonexistent path

Change-Id: Id1dfb65cb54a62983953b1493afa2b6735802a93
diff --git a/settings.gradle b/settings.gradle
index 3633cb2..f3f0ccc 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -29,10 +29,13 @@
 
     def file
     if (filePath instanceof String) {
-        file = new File(filePath)
+        file = new File(rootDir, filePath)
     } else {
         file = filePath
     }
+    if (!file.exists()) {
+        throw new Exception("Path " + file + " does not exist; cannot include project " + name)
+    }
     project(name).projectDir = file
 }