Clarify a bit the Mediapreparer download failure

Include path and make sure to point at host machine so
it's clear it's not device related.

Test: unit tests
Bug: 155877636
Change-Id: If796afd4ef8a26bc05c425bfabe0f6808116e3a7
1 file changed
tree: 757f1c14f26c9bacb79020dd9adff55e65478f5a
  1. build/
  2. common/
  3. tools/
  4. OWNERS
  5. PREUPLOAD.cfg
  6. test_defs.sh