worker: Log which outputs are missing when task is unexpectedly incomplete

[progval]

Motivation and Context

When a task ran but did not write all its outputs, Luigi errors with:

RuntimeError: Unfulfilled dependency at run time: NodeProperties_graph__tmp_pytest_of_d_cnt_dir_rev_rel__02a93cea21

which can be hard to debug for tasks with many outputs. After this PR, it lists which outputs are missing. For example:

RuntimeError: Unfulfilled dependency at run time: NodeProperties_graph__tmp_pytest_of_d_cnt_dir_rev_rel__02a93cea21 (/tmp/pytest-of-dev/pytest-512/test_compressgraph_None_0/compressed_graph/graph.property.committer_timestamp.bin, /tmp/pytest-of-dev/pytest-512/test_compressgraph_None_0/compressed_graph/graph.property.message.bin, /tmp/pytest-of-dev/pytest-512/test_compressgraph_None_0/compressed_graph/graph.property.tag_name.bin, /tmp/pytest-of-dev/pytest-512/test_compressgraph_None_0/compressed_graph/graph.property.content.is_skipped.bin, /tmp/pytest-of-dev/pytest-512/test_compressgraph_None_0/compressed_graph/graph.property.tag_name.offset.bin, /tmp/pytest-of-dev/pytest-512/test_compressgraph_None_0/compressed_graph/graph.property.committer_id.bin, /tmp/pytest-of-dev/pytest-512/test_compressgraph_None_0/compressed_graph/graph.property.content.length.bin, /tmp/pytest-of-dev/pytest-512/test_compressgraph_None_0/compressed_graph/graph.property.author_id.bin, /tmp/pytest-of-dev/pytest-512/test_compressgraph_None_0/compressed_graph/graph.property.message.offset.bin, /tmp/pytest-of-dev/pytest-512/test_compressgraph_None_0/compressed_graph/graph.property.committer_timestamp_offset.bin)

Description

I implemented a basic __str__ for all target types, using either the path if it's filesystem-like, or a table name for database-like targets (to be consistent with BigQueryTarget).

Have you tested this? If so, how?

Unit tests are not broken, and this works for me (only tested with local filesystem)