Make path_size_megabytes() more robust.

[concretevitamin]

Saw some parsing errors that threw ValueError.

This change

• uses regex to look for 'total size is ...'
• on error, return -1 as estimated size, as it's used for warnings only

Tested (run the relevant ones):

• Any manual or new tests for this PR (please specify below)
  • sky launch test.yaml with workdir: .; checked that sizes are correct
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[Michaelvll]

Thanks for the changes @concretevitamin! LGTM. Some questions