improve OTA failure messages

Replace OTA script constructs of the form:

   assert(foo);

with

   foo || abort("sensible message");

so that the log and the on-screen display is somewhat more accessible
to non-experts.  (assert() displays the source code of the false
expression 'foo'.)

Change-Id: Ic99448e4466561d305b167cd4d5c1f0f2dbadcce
diff --git a/tools/releasetools/ota_from_target_files b/tools/releasetools/ota_from_target_files
index e0d5d91..1e1d04e 100755
--- a/tools/releasetools/ota_from_target_files
+++ b/tools/releasetools/ota_from_target_files
@@ -387,7 +387,8 @@
 
   if not OPTIONS.omit_prereq:
     ts = GetBuildProp("ro.build.date.utc", OPTIONS.info_dict)
-    script.AssertOlderBuild(ts)
+    ts_text = GetBuildProp("ro.build.date", OPTIONS.info_dict)
+    script.AssertOlderBuild(ts, ts_text)
 
   AppendAssertions(script, OPTIONS.info_dict)
   device_specific.FullOTA_Assertions()