Print a more useful error message when the target binary can't be removed.

Print a more useful error message when the target binary can't be removed.

This happens sometimes on Windows if the target is still running, and right
now cargo prints a very cryptic message:
```
$ cargo build
   Compiling sccache v0.1.0 (file:///C:/build/sccache2)
An unknown error occurred
```

With this patch we get a much more useful error:
```
$ ../cargo/target/debug/cargo build
   Compiling sccache v0.1.0 (file:///C:/build/sccache2)
error: Could not remove file: c:\build\sccache2\target\debug\sccache.exe.

To learn more, run the command again with --verbose.
```