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