upload_symbols: add more logging when finishing up
upload_symbols: add more logging when finishing up

Add a few log messages at critical points to help track down hangs.

BUG=chromium:355843
TEST=`./scripts/upload_symbols_unittest.py` passed

Change-Id: Ic63b7fdcf3f70613a086160b474529d19ef42cdf
Reviewed-on: https://chromium-review.googlesource.com/191380
Reviewed-by: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite