rustc: print filename if file cannot be written

File cannot be written, for example, if directory does not exist.

Before this commit:

```
% rustc -o nonexistent/program program.rs
error: could not write output: No such file or directory
```

With this commit:

```
% rustc -o nonexistent/program program.rs
error: could not write output to nonexistent/program.0.o: No such file or directory
```

This is useful when full rust command is not displayed, or when last
error is preceded by thousands of warnings.

0 files changed, 0 insertions, 0 deletions