refactor: Improve error message for docker failures

Author: pgrzesik

lib/pip.js

@@ -412,6 +412,14 @@ async function installRequirements(targetFolder, pluginInstance) {
             'PYTHON_REQUIREMENTS_COMMAND_NOT_FOUND'
           );
         }
+
+        if (cmd === 'docker' && e.stderrBuffer) {
+          throw new pluginInstance.serverless.classes.Error(
+            `Running ${cmd} failed with: "${e.stderrBuffer.toString().trim()}"`,
+            'PYTHON_REQUIREMENTS_DOCKER_COMMAND_FAILED'
+          );
+        }
+
         if (log) {
           log.info(`Stdout: ${e.stdoutBuffer}`);
           log.info(`Stderr: ${e.stderrBuffer}`);