fix: improve command device support

Propagate inboundHandler errors better.  Prevent asynchronous
rejection errors.

packages/SwingSet/src/devices/command-src.js

@@ -21,7 +21,7 @@ export default function setup(syscall, state, helpers, endowments) {
         const body = JSON.parse(`${bodyString}`);
         SO(inboundHandler).inbound(Nat(count), body);
       } catch (e) {
-        console.log(`error during inboundCallback: ${e}`);
+        console.log(`error during inboundCallback`, e);
         throw new Error(`error during inboundCallback: ${e}`);
       }
     });
@@ -33,6 +33,10 @@ export default function setup(syscall, state, helpers, endowments) {
       },
 
       sendResponse(count, isReject, obj) {
+        if (isReject && obj instanceof Error) {
+          console.log('inboundHandler rejected with:', obj);
+          obj = { message: obj.message };
+        }
         try {
           deliverResponse(count, isReject, JSON.stringify(obj));
         } catch (e) {

packages/cosmic-swingset/lib/ag-solo/start.js

@@ -137,10 +137,19 @@ async function buildSwingset(
     // this promise could take an arbitrarily long time to resolve, so don't
     // wait on it
     const p = cm.inboundCommand(obj);
+
+    // Register a handler so that we don't get complaints about
+    // asynchronously-handled callbacks.
+    p.catch(_ => {});
+
     // TODO: synchronize this somehow, make sure it doesn't overlap with the
-    // processKernel() call in deliverInbound()
+    // processKernel() call in deliverInboundToMbx()
     await processKernel();
-    return p;
+
+    // Rethrow any exception so that our caller must handle it.
+    return p.catch(e => {
+      throw e;
+    });
   }
 
   const intervalMillis = 1200;