cli: Provide better error message behavior when prepare fails

Author: cassava

cli/cloe_launch/__main__.py

@@ -342,7 +342,13 @@ def cli_prepare(
     engine.conan_options = list(conan_option)
     engine.conan_settings = list(conan_setting)
 
-    engine.prepare()
+    try:
+        engine.prepare()
+    except ChildProcessError:
+        # Most likely scenario:
+        # 1. conan had an error and terminated with non-zero error
+        # 2. error has already been logged
+        sys.exit(1)
 
 
 # _________________________________________________________________________

cli/cloe_launch/utility.py

@@ -25,7 +25,8 @@ def run_cmd(
     )
     if result.returncode != 0:
         logging.error(f"Error running: {' '.join(cmd)}")
-        logging.error(result.stdout)
+        if result.stdout is not None:
+            logging.error(result.stdout)
         if must_succeed:
             raise ChildProcessError()
     return result