Use anyhow::Result in xtask, add contexts

This builds on #2231 but was actually done before that. You see, the
cause for #2231 was that I got this error message:

    Error: Error { kind: Io(Os { code: 2, kind: NotFound, message: "No such file or directory" }) }

Just switching to `anyhow::Result` got me stack traces (when setting
`RUST_LIB_BACKTRACE=1`) that at least showed

    stack backtrace:
      0: std::backtrace::Backtrace::create
      1: std::backtrace::Backtrace::capture
      2: anyhow::error::<impl core::convert::From<E> for anyhow::Error>::from
      3: xtask::install_server
      4: xtask::install
      5: xtask::main
      6: std::rt::lang_start::{{closure}}
      7: std::panicking::try::do_call
      8: __rust_maybe_catch_panic
      9: std::rt::lang_start_internal
      10: std::rt::lang_start
      11: main

With the added contexts (not at all exhaustive), the error became

    Error: install server

    Caused by:
        0: build AutoCfg with target directory
        1: No such file or directory (os error 2)

Since anyhow is such a small thing (no new transitive dependencies!),
and in general gives you `Result<T, Box<dyn Error>>` on steroids, I
think this a nice small change. The only slightly annoying thing was to
replace all the `Err(format!(…))?` calls (haven't even looked at whether
we can make it support wrapping strings though), but the `bail!` macro
is shorter anyway :)

1 files changed, 2 insertions, 2 deletions

diff --git a/xtask/src/bin/pre-commit.rs b/xtask/src/bin/pre-commit.rs
index cc6ccb25e..44507fb74 100644
--- a/xtask/src/bin/pre-commit.rs
+++ b/xtask/src/bin/pre-commit.rs
@@ -19,10 +19,10 @@ fn update_staged() -> Result<()> {
         .current_dir(&root)
         .output()?;
     if !output.status.success() {
-        Err(format!(
+        anyhow::bail!(
             "`git diff --diff-filter=MAR --name-only --cached` exited with {}",
             output.status
-        ))?;
+        );
     }
     for line in String::from_utf8(output.stdout)?.lines() {
         run(&format!("git update-index --add {}", root.join(line).to_string_lossy()), ".")?;