sbox: best-effort copy output files on failure

Error messages printed by failing commands may reference output files
that were created by the command, for example printing a command line
to copy and paste to update a baseline file.  Copy output files in the
sandbox to their final locations, ignoring missing files, so that the
messages are valid.

Bug: 185516277
Test: m out/soong/.intermediates/frameworks/base/system-api-stubs-docs-non-updatable/android_common/metalava/api_lint.timestamp with lint error
Change-Id: I604a11c9b54e409ca5bc5c016cd04b3133f74a60