update_crate_tests: fail gracefully

Allow this script to accept a path to the crate being updated.
This allows it to be run separately from cargo2android.

Also, fail gracefully on error. TEST_MAPPING update failures should
not block crate updates.

Test: tools/external_updater/updater.sh update rust/crates/libc
Bug: 179132533
Change-Id: I3a4229f479ab3d2793df16b470f0a0632b9ee495
2 files changed