Copy crash log artifact from Cloud Storage results bucket to local results directory

[AaronMT]

Currently, Flank copies/generates:

• CostReport.txt
• FullJUnitReport.xml
• JUnitReport.xml
• MatrixResultsReport.txt
• matrix_ids.json
• session_id.txt

For a Robo Test run on Android, I recently noticed Firebase team is now generating a

matrix_0/<device>/data_app_crash_0_<package>.txt file highly relevant crash stack information.

It would be super useful for Flank to copy this out to local results when it's available.