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, 7 insertions, 0 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 97452951c..c9ab3ef10 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -9,6 +9,11 @@ dependencies = [
 ]
 
 [[package]]
+name = "anyhow"
+version = "1.0.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
 name = "arrayvec"
 version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -1823,6 +1828,7 @@ dependencies = [
 name = "xtask"
 version = "0.1.0"
 dependencies = [
+ "anyhow 1.0.19 (registry+https://github.com/rust-lang/crates.io-index)",
  "autocfg 0.1.7 (registry+https://github.com/rust-lang/crates.io-index)",
  "pico-args 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "proc-macro2 1.0.6 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -1847,6 +1853,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [metadata]
 "checksum aho-corasick 0.7.6 (registry+https://github.com/rust-lang/crates.io-index)" = "58fb5e95d83b38284460a5fda7d6470aa0b8844d283a0b614b8535e880800d2d"
+"checksum anyhow 1.0.19 (registry+https://github.com/rust-lang/crates.io-index)" = "57114fc2a6cc374bce195d3482057c846e706d252ff3604363449695684d7a0d"
 "checksum arrayvec 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)" = "cff77d8686867eceff3105329d4698d96c2391c176d5d03adc90c7389162b5b8"
 "checksum atty 0.2.13 (registry+https://github.com/rust-lang/crates.io-index)" = "1803c647a3ec87095e7ae7acfca019e98de5ec9a7d01343f611cf3152ed71a90"
 "checksum autocfg 0.1.7 (registry+https://github.com/rust-lang/crates.io-index)" = "1d49d90015b3c36167a20fe2810c5cd875ad504b39cff3d4eae7977e6b7c1cb2"