refactor(build.rs): handle errors without panic

This has no real difference to the user, just makes it easier to find
uses of .expect with grep.

Signed-off-by: Lars Wirzenius <liw@liw.fi>

build.rs

@@ -23,5 +23,8 @@ fn main() {
     println!("cargo:rustc-env=GIT_HEAD={hash}");
     println!("cargo:rustc-rerun-if-changed=.git/HEAD");
 
-    subplot_build::codegen("ci-broker.subplot").expect("failed to generate code with Subplot");
+    if let Err(e) = subplot_build::codegen("ci-broker.subplot") {
+        eprintln!("failed to generate code with Subplot: {e}");
+        std::process::exit(1);
+    }
 }