Improve build errors when postgres_ffi fails (#2460)

This commit does two things of note:

 1. Bumps the bindgen dependency from `0.59.1` to `0.60.1`. This gets us
    an actual error type from bindgen, so we can display what's wrong.
 2. Adds `anyhow` as a build dependency, so our error message can be
    prettier. It's already used heavily elsewhere in the crates in this
    repo, so I figured the fact it's a build dependency doesn't matter
    much.

I ran into this from running `cargo <cmd>` without running `make` first.
Here's a comparison of the compiler output in those two cases.

Before this commit:

```
error: failed to run custom build command for `postgres_ffi v0.1.0 ($repo_path/libs/postgres_ffi)`

Caused by:
  process didn't exit successfully: `$repo_path/target/debug/build/postgres_ffi-2f7253b3ad3ca840/build-script-build` (exit status: 101)
  --- stdout
  cargo:rerun-if-changed=bindgen_deps.h

  --- stderr
  bindgen_deps.h:7:10: fatal error: 'c.h' file not found
  bindgen_deps.h:7:10: fatal error: 'c.h' file not found, err: true
  thread 'main' panicked at 'Unable to generate bindings: ()', libs/postgres_ffi/build.rs:135:14
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

After this commit:

```
error: failed to run custom build command for `postgres_ffi v0.1.0 ($repo_path/libs/postgres_ffi)`

Caused by:
  process didn't exit successfully: `$repo_path/target/debug/build/postgres_ffi-e01fb59602596748/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-changed=bindgen_deps.h

  --- stderr
  bindgen_deps.h:7:10: fatal error: 'c.h' file not found
  Error: Unable to generate bindings

  Caused by:
      clang diagnosed error: bindgen_deps.h:7:10: fatal error: 'c.h' file not found
```

libs/postgres_ffi/Cargo.toml

@@ -25,4 +25,5 @@ postgres = { git = "https://github.com/neondatabase/rust-postgres.git", rev="d05
 wal_craft = { path = "wal_craft" }
 
 [build-dependencies]
-bindgen = "0.59.1"
+anyhow = "1.0"
+bindgen = "0.60.1"