rustc_incremental: avoid ICE when dumping dep graph fails

[cuishuang]

-Zdump-dep-graph used unwrap() when creating or writing the dependency graph output files. As a result, setting RUST_DEP_GRAPH to a path whose parent directory does not exist, or to another unwritable location, caused rustc to ICE.

This changes dump_graph to propagate filesystem errors and emit the existing structured file-writing diagnostic at the caller.

The run-make test now covers an invalid output directory and verifies that rustc reports a normal error without ICEing.

Tests:

• ./x test tests/run-make/dep-graph
• ./x fmt --check
• ./x test tidy

[rustbot]

r? @wesleywiser

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[rust-bors]

☔ The latest upstream changes (presumably #160102) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.