chore(pacmak): improve error messaging (#1817)

When a shelled-out process fails (either on a non-0 status, or killed by
a signal), the error message did not include the command that just
failed, causing ambiguity with respects to what happened. Additionally,
the output included STDOUT and STDERR concatenated (in this order)
without any way to distinguishe which is which... This adds a prefix
string to each line of those to denote whether STDOUT or STDERR.

.eslintrc.yaml

@@ -0,0 +1,3 @@
+---
+extends: ./eslint-config.yaml
+root: true

packages/jsii-pacmak/lib/util.ts

@@ -73,9 +73,20 @@ export async function shell(
         const command = `${cmd} ${args.join(' ')}`;
         return ko(
           new Error(
-            `Command exited with ${reason}:\n- Command: ${command}\n- STDOUT:\n${out}\n- STDERR:\n${err}`,
+            [
+              `Command (${command}) failed with ${reason}:`,
+              prefix(out, '#STDOUT> '),
+              prefix(err, '#STDERR> '),
+            ].join('\n'),
           ),
         );
+
+        function prefix(text: string, add: string): string {
+          return text
+            .split('\n')
+            .map((line) => `${add}${line}`)
+            .join('\n');
+        }
       });
     });
   }