gen/pb-rust: more errors in build script

Signed-off-by: Andrew Pan <andrew.pan@trailofbits.com>

gen/pb-rust/sigstore-protobuf-specs/build.rs

@@ -1,9 +1,16 @@
+use anyhow::Context;
+
 /// Find the standard protobuf include directory.
 fn protobuf_include_path() -> String {
     let mut protobuf_root = which::which("protoc")
-        .ok()
+        .context("couldn't find protoc!")
         // dirname(/bin/protoc) / ../
-        .and_then(|path| path.ancestors().nth(2).map(|p| p.to_path_buf()))
+        .and_then(|path| {
+            path.ancestors()
+                .nth(2)
+                .map(|p| p.to_path_buf())
+                .with_context(|| format!("couldn't traverse path: {path:?}"))
+        })
         .expect("protobuf installation directory not found!");
     protobuf_root.push("include");
     protobuf_root.to_str().unwrap().to_owned()