chore: code review feedback.

wdl-engine/src/backend/local.rs

@@ -18,6 +18,7 @@ use futures::future::BoxFuture;
 use tokio::process::Command;
 use tokio::sync::Semaphore;
 use tracing::info;
+use tracing::warn;
 use wdl_analysis::types::PrimitiveType;
 use wdl_ast::v1::TASK_REQUIREMENT_CPU;
 use wdl_ast::v1::TASK_REQUIREMENT_MEMORY;
@@ -294,8 +295,15 @@ impl LocalTaskExecutionBackend {
     /// If `max_concurrency` is `None`, the default available parallelism for
     /// the host will be used (typically the logical CPU count).
     pub fn new(max_concurrency: Option<usize>) -> Self {
-        let max_concurrency =
-            max_concurrency.unwrap_or_else(|| available_parallelism().map(Into::into).unwrap_or(1));
+        let max_concurrency = max_concurrency.unwrap_or_else(|| {
+            available_parallelism().map(Into::into).unwrap_or_else(|_| {
+                warn!(
+                    "unable to determine available parallelism: tasks will not be executed \
+                     concurrently"
+                );
+                1
+            })
+        });
         Self {
             lock: Semaphore::new(max_concurrency).into(),
             max_concurrency,

wdl-engine/src/eval/v1/task.rs

@@ -253,7 +253,11 @@ impl<'a> TaskEvaluator<'a> {
 
         let ast = match document.node().ast() {
             Ast::V1(ast) => ast,
-            _ => return Err(anyhow!("document is not a 1.x document").into()),
+            _ => {
+                return Err(
+                    anyhow!("task evaluation is only supported for WDL 1.x documents").into(),
+                );
+            }
         };
 
         // Find the task in the AST

wdl-engine/src/eval/v1/workflow.rs

@@ -1235,7 +1235,11 @@ impl WorkflowEvaluator {
 
         let ast = match document.node().ast() {
             Ast::V1(ast) => ast,
-            _ => return Err(anyhow!("document is not a 1.x document").into()),
+            _ => {
+                return Err(
+                    anyhow!("workflow evaluation is only supported for WDL 1.x documents").into(),
+                );
+            }
         };
 
         info!(