standalone: be more helpful with bad block device configs (#758)

Author: iximeow

bin/propolis-standalone/src/config.rs

@@ -136,7 +136,9 @@ pub fn block_backend(
     log: &slog::Logger,
 ) -> (Arc<dyn block::Backend>, String) {
     let backend_name = dev.options.get("block_dev").unwrap().as_str().unwrap();
-    let be = config.block_devs.get(backend_name).unwrap();
+    let Some(be) = config.block_devs.get(backend_name) else {
+        panic!("No configured block device named \"{}\"", backend_name);
+    };
     let opts = block::BackendOpts {
         block_size: be.block_opts.block_size,
         read_only: be.block_opts.read_only,
@@ -149,7 +151,14 @@ pub fn block_backend(
 
             // Check if raw device is being used and gripe if it isn't
             let meta = std::fs::metadata(&parsed.path)
+                .with_context(|| {
+                    format!(
+                        "opening {} for block device \"{}\"",
+                        parsed.path, backend_name
+                    )
+                })
                 .expect("file device path is valid");
+
             if meta.file_type().is_block_device() {
                 slog::warn!(log, "Block backend using standard device rather than raw";
                     "path" => &parsed.path);