releasetools: Capture stderr output when calling delta_generator.

Prior to this CL, the call to delta_generator in
check_ota_package_signature.VerifyAbOtaPayload() didn't redirect stderr.
The logs (mostly INFO) on successful verification added noise to the
normal output, which also upset the unittest result parser.

This CL captures stderr outputs from delta_generator, and will only dump
them on error.

Bug: 72884343
Test: `python -m unittest -v test_ota_from_target_files > /dev/null`
      gives clean output.
Test: Inject error into delta_generator. The call to
      check_ota_package_signature correctly dumps both of stdout and
      stderr outputs.
Change-Id: I014a4b21bf758dcf0a4b9963259d6019851935ee